Here are a few tips I've come to after many mistakes. Save yourself some cash and read this first.
Size  Decide what your trying to accomplish, then try to minimize the size all you can. This will save you serious dough on wasted space.
Shape  Your fingers have tendons under stress. They are your friends. Don't make corners that will mash them. It hurts.
Texture  After 5 months of putting manufactured holds under the microscope to copy their texture, I think I finally figured it out. There are several opinions on a good texture to make your hands stick, so I'll explain a few later.
Usability  Make the most of each hold. Molds are an expensive part, so making a hold that can be used in different directions really helps.
Stability  Look at the pressure areas of your hold. Make these areas, such as around the bolt, well supported so they won't crack when torqued. I've also learned of a foam backing to stop slippage and prevent shaving off the texture on the wall.

Disclaimer! Polyurethane foam shavings can be very harmful to your lungs and sinuses if breathed in (ie. nosebleeds, sinus infections, and lung tissue damage)! WEAR A MASK to avoid breathing in the small shavings! If you are sculpting with foam, grab a kitchen knife and a pencil. These with your fingers will be your tools. Otherwise, grab a clump of clay, wet your hands, and started shaping your rock. For your first try, you might want to make a crimper since small mistakes cost less. I suggest using a flat surface to sculpt on to keep the back flat.

Now, make an indention of the bolt head by pressing the head (not the threads) into to clay. Make this only as deep as the bolt head and at least 1/4" from the base. Now lightly move the bolt head in circles to widen the hole and make placing your bolt on the hold easier later. When this is done, flip the bolt over and using the smaller diameter of the threaded portion of the bolt, press a small indention (if you do not own a drill, press it all the way to the base). If this confuses you read "Washer Alignment" in the Emailed Tips & FAQ page. Round off the sharp edge of the lip for your bolt hole. When this is finished, you are ready to add texture.

As I mentioned before, there are several opinions on texture. I've found that ultimately, Polyester Resin is usually too smooth for sweaty hands. Some suggest cramming sand onto the outside of the clay prototype for a grit feel. Others paint a resin later on the clay model and fix sand into this. For green foam carvers, using a wire brush is good and even simply breaking off pieces of the oasis and using the jagged torn portion is great. Also, the surface of the oasis makes some texture on its own. For flakes and other flat pieces, breaking the edges off with a hammer gives you the absolute best possible texture Bondo can offer. Please try these to find what you like.

I'm currently trying a new concept. After seeing that manufactured holds actually have a surface with millions of little pits, I'm copying this. For this guide, I'll start you out with this method. Get out your kitchen brush and lightly tap the bristle heads onto the moist clay surface. When you see the surface become saturated with pits, you're done. For a finer feel, use thinner bristles (toothbrush?). Please let me know what ideas you find in this area!

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