Cleaning Up
A handheld restaurant table sweeper that can be purchased at a restaurant supply store is great for picking up things that fall on the floor. It consists of a white handheld box with a rotating brush and a lid. After I drop my pearls, beads, little balls of clay, whatever, I run the sweeper over the area, then I flip the device over, remove the lip and I can retrieve all of the items I lost.
~ Holly

For delicate pieces that you need to continue to work on, but keep breaking either because your a klutz or it is delicate. Repair the break and finish that component and put in a craft only toaster oven and bake on a high temp anywhere between 300 - 500 degrees. It makes it nice and sturdy. The downfall is you can not do any more pre-finishing on that part, so make sure you do all greenware sanding before the toaster.
~ Holly Gage

Replacing a Cracked or Ruined Gem in a Finished Piece (See Setting A Gem in a Fired Metal Clay Piece with Oil Paste)

Use a weighted cup for your water. I was using an old plastic pill container and spilled it at my bench more times than I'd like to admit, however I have never spilled my new and improved container that is weighted on the bottom.

Leaves and Paste
To prevent curling. A good rule of thumb for leaves of average pendant size - approx 4cm x 2.5cm - is to weigh the leaf periodically as it is being pasted. Should weigh at least 3.5 grams before firing, and make sure edges are thick, and pronounced middle ribs are well covered! Try and find leaves that have "substance" - in other words, they must not be juicy soft like spinach! Try mint leaves for terrific texture, and wonderful smell when fired. When starting out to do leaves, avoid oily ones like geraniums until you know how to get over the fact that they DONT like being pasted - the oils repel the clay. Hairy leaves are also a big problem - these are best rolled - impressed - into the clay to give the vein texture. Look for tiny leaves to paint and keep for adding to other creations, often can make a so-so piece into a winner!
- Dragonscm

Many leaves curl during firing. This seems to be due to the thickness of the silver clay paste (thinner pieces curl more) and the type of leaf coated. Some types of leaves tend to curl more than others when burned. Uneven coatings of silver clay could also add to the curling effect. As long as they leaves are not too-too thin and were fired at the higher MC temperatures, you can gently uncurl them with your fingers or other tool. If they start to crack, you should stop. In that case, you could either re-fire for longer/hotter or add more clay and refire for longer/hotter. If they were only fired at low fire mc temperatures, you should re-fire at a higher temperature before trying to uncurl them very much. I hope that is reasonably clear.
Mary Ellin D'Agostino, http://www.medacreations.com

The problem with slip-coating is not in the leaf... it's in our natural impatience to want something beautiful NOW! LOL! Successful slip coating is best accomplished with 1. A totally dried organic material, 2. Absolute minimum of 10 coats, graduating the slip from thin to medium thickness; 3. Totally drying between coats; thus indicating a period of a week-10 days to be absolutely sure you've applied enough coats, drying properly..... curling will be minimal
Sarah Triton

Someone mentioned using the food wrap that has the slightly sticky surface. Place the front of the leaf on the sticky side and press firmly together. I tried that with a leaf and it worked great. No curling while pasting and it fired great.
Lorrene J. Davis, http://www.freewebs.com/ljdavisdesigns/index.htm

Liver of Sulfur
Keep a small film container full of dry LOS so that you do not have to keep exposing your large container to light and air just to get a single piece out each time.
Gordon Uyehara, http://home.hawaii.rr.com/energies/

Liver of Sulfur - Beauty and the Beastly Smell is a comphrensive article that has recipes and answers to some of the most frequently asked questions people have about using LOS. With a bit of knowledge under your belt, you will feel much less intimidated to use LOS, and once you realize that a LOS patinas CAN be removed, you will feel more free to experiment without the worry if ruining a piece you spent hours to create.
Holly Gage

Mixed Clay Types
When using two different types of clay in one project always fire at the higher firing temperature of the two clays. This would also hold true when using the two different brands of metal clay, Art Clay and PMC.
Holly Gage

Knead together well. The shrinkage of PMC Standard is quite a bit
different. Could create some interesting effects if not mixed thoroughly.
Gordon Uyehara, http://home.hawaii.rr.com/energies/

Molding
There is a good 2-part mold material called Amazing Mold Putty. You can get it at Michael's and online. I really like the details
~ Judi Weer

I use Alley Goop. That has been my favorite for a long time. Alley Goop is only sold on line as far as I know.
~ Jean Melton

I really like the two-part mold mix they have at Art Clay World -very easy to use, it is "soft" rubber, and so second runner up if a harder rubber mold is needed is the Sculpty mold maker, which is 1 part.
~ Liz Hall, http://www.lizardsjewelry.com

There is a two part cold molding compound called Belicone Rubber which sets up completely in about 30 mins. from Rio Grande (online and catalog). Also, there is Sculptey Make and Bend, which you mold and then bake. The thing I like about this is there is no time limit. So a technique I have developed is similar to tear away technique, but instead of a photocopy use a pencil drawing. With Sculptey bake and bend the result is slightly different, you get the pencil drawing transferred onto the Sculptey and the pencil lines are raised - hard to explain, but it is not grainy and flat like in the regular tear away technique. You can bake it as is or use the pencil lines as a guide. Then you have all the time you need to take a tool or stylus and push the Sculptey around and make low relief areas, textures, lines what ever - remember you are working backward and any area you push in will be raised.
~ Holly Gage

Placeholders
Used for stones that cannot be fired in place
Options
Delight Paper Clay burns away; this can be used as a temporary armature.
Creative Paper Clay does not burn away, this is stable enough to chip away if need be or removed and used again if thick enough.
Uses:
It can be used as a placeholder for a fine silver bezel embedded in the Metal Clay. It helps keep the alignment of the bezel and also is helpful for minimizing distortion caused by Metal Clay shrinkage during firing. You can gently pack the bezel with the paper clay or mold a duplicate of the stone. I recommend you dry it completely, but once I did it in a hurry and it was only 3/4 dry and it worked.
It can also be used as a placeholder for ring sizing
Pros:
Easy to mold and it can be shaped and formed in a ready-made form or mold. Can be dried on a hot plate. No shrinkage. Can survive the pressure of shrinkage.
Cons:
Limited use of 1 to 4 times.
It can dry out. Kneading water into it will revive it as long as it is not bone dry. Put a damp paper towel in the plastic package to keep it moist.
~ Holly Gage

Other Cons:
I found that it was difficult to get an accurate model of the stone--something pourable like investment or plaster is much easier to work with. I found carving much too time consuming and difficult to accurately reproduce the stone
Mary Ellin D'Agostino, http://www.medacreations.com

Plaster of Paris
Plaster of Paris is used to make a model of the stone...it's cheap and you can quench the piece right out of the kiln and the P of P dissolves. Can’t be used with a torch, though. Do it outside, it's messy to mix up.
~ Sarah Triton

Tim explains placeholders in his book "Working with Precious Metal Clay". You duplicate the stone in its original size (shape, height etc.) in plaster. He suggests to poor the plaster into a paper cup the height of the stone, but you can do this on any small container close to the size of your stone so as not to use too much plaster and have to carve tons away afterwards. Then, once dry (at least overnight), you carve a duplicate of your stone. You will use this in your design as you would a ring plug, leaving enough clay around it so the clay will shrink but not break during shrinkage since it will be stressed upon achieving the size of the plug. If you're planning on a bezel, then you can calculate the bezel (and space to be left for the stone) in your design by enlarging a photocopy of your stone by the percentage of shrinkage of the clay you are using, and make the bezel the enlarged size. You position your plug, centered, inside that bezel so the clay shrinks around it but stops at the size of your stone. Some extra tips for duplicating the stone are:
1- drawing around it on a piece of paper then using that as your guide to cut the plaster, once close to the stone shape, put them back to back until you make sure you've got it right (“back to back" only for a symmetric shaped stone, not good for an odd shaped stone where, if you put it back to back, you'll get it's mirror shape)
2- measure the height with a caliper and check your plaster model.

Another possibility is making a print or a "well" with the actual stone directly onto the metal clay, then taking it out very carefully (ideally after dry, and having left a back hole to push the stone out) filling that with plaster (for the back hole you can just put a little piece of paper so it won't drool out, which will burn during firing). But you'll still have to make your design around it in a way that the stone will be held in place either with prongs, or with coils or something since the hole that's left after firing should be the size of the stone, so it can pop out if you don't integrate some sort of bezel/prong/coil etc. into your design. Don't forget to leave room to account for shrinkage here too, and also to make it deep enough.
~ Angela B. Crispin, http://www.LAngeEstLa.com

 I like Tim's idea of mixing the plaster in little zip lock bags and sniping off one of the corners so you can squirt out the plaster like a pastry bag.
Gordon Uyehara, http://home.hawaii.rr.com/energies/

Cons:
It all depends on the plaster. Some brands/types may work ok, others will not. I've had some pretty dismal failures with plaster. For example, when using ring patties made out of plaster, the end result was a smaller ring that the desired size--the plaster did shrink. 
Mary Ellin D'Agostino, http://www.medacreations.com

High Temperature Investment is another option.
Silicica-free investment available at
http://www.CoolTools.us
http://www.medacreations.com.
Make a mold of the stone using two-part silicone putty mix - Belicold or any brand that makes a flexible mold. Flexible is important. I grease up the mold and pour investment into the mold. The same investment I use for making ring plugs. I set the plug in the bezel as a placeholder and fire.
~ Carol Augustine

Plasticine modeling clay is infinitely re-usable and if you don't get a good impression of the stone on the first try, you can easily try again. You do need to make the model a tiny bit bigger than the real stone, so I use 2 layers of plastic wrap on small stones--wrap the stone in two layers of plastic wrap and twist around the top of the stone to get a form fitting spacer around the stone. Press into the modeling clay carefully to the level of the table to create an accurate model of a faceted stone. You can purchase my complete directions that detail all the ins and outs of the process on my website at under kits or under books.
Mary Ellin D'Agostino, http://www.medacreations.com

The mold you make in the clay/plasticine (Dick Blick modeling clay) isn't meant to be permanent. It is only used to make an impression of the stone, element; you want to place in your clay during firing. Once the investment is hard, you peel the clay away and use it (the clay) again later to make another temporary mold for an investment mock up of a stone, element, etc.
~ Laura Hastings

Polishing (also see finishing, prefinishing, and tumbling, White on the Surface of Metal Clay)
After firing.
For more aggressive polishing use 3mm rubber wheels made for silver and a flex shaft or dremel. Try using Medium for "really" aggressive work, Fine for a small imperfection, and very fine for the final high polish. The Coarse grit for me is just too aggressive and even the Medium can be very aggressive for the fine silver so keep an eye out. The key is to keep the wheel moving or you will get grooves and wheel lines in the silver. Depending on how deep the imperfections are. A final tumble in a tumbler with steel shot or a polishing pad will give it a nice even look. This is my secret weapon for a mirror finish.
~ Holly Gage

Rollers and Rolling Clay Out
Uniform clay thickness. Pinzart.com -- Pam East's slats: the slats are color coded to the thickness: measurements are in inches, mm and cards.
Jerri Duncan

Rollers. A piece of PVC pipe is an easy tool to get for rolling out slabs. But back in my pottery days, I was well aware that rolling pins are not all created equal. I had dozens of them for different jobs. Subtle bearing movement was critical for a clean roll. So too with PMC. I have always used the hard rubber printing brayers. They don't cost much, will last you a life time, and the handle makes all the difference in the world when it comes to the control over the thickness and evenness of the slab. (You are rolling with your wrist, not your hand.) The trick is to not try to roll flat all at once. Roll a few times lightly in one direction, never rolling off the edge, but leaving a fat lip. Pick the piece up, flip it upside down, and horizontally. Roll a few more times, LIGHTLY, flip again and turn vertically, repeat. By moving it around your work surface and flipping it, you never have to grease the clay, brayer or your surface. I don't grease anything, but my hands, ever. The minute your brayer picks up any wet clay, stop, wash it off and dry. Once it has any bit of wet clay on it, it will keep picking up wet clay.

Though I have tried non-stick surfaces, which I did have to grease, I still keep coming back to good old freezer paper, shiny side up. When one part of the paper starts to show signs of getting wet or sticky, I move to a drier part, and let it dry out. Then once I have finished an especially delicate piece, instead of trying to get it off the surface, I just leave it alone. When it is dry it will just peel off the paper, but if I am in a hurry, I can cut the paper around the shape, so I never risk distortion, and set the paper on my coffee cup heater, or place on a pan and put in the oven. When some students have done especially fine syringe work, I just place it in the kiln on the paper, and let it burn up.

Though I don't know this for a fact, I suspect that over greasing a piece has a few disadvantages. As you cut out pieces, wade up the excess to reuse, it becomes difficult to know if the clay is still actually wet enough, or if it just seems wet because it is so greasy. (You will find out the difference soon enough if you then use that clay to try to roll a coil, it will look wet, but keep cracking. ) If you are rolling out flat pieces you are just texturing, it isn't much of a problem. But if you are fabricating, I think eventually the grease is going to become a barrier to a good connection.
Elizabeth R. Agte, http://www.agte.com