Found this:
http://www.stephenwilliamson.com/Climbi ... gholds.htm
pretty interesting.
One note: one of the more expensive parts of making holds is making the mold, unless you reuse one mold over and over and over again, it probably won't be worth while. One friend I have uses play-dough for some of his molds, he rests the playdough in sand so it won't just flop appart when you put the resin in. He has also done things like used half a small basketball, where you can kinda warp the shape of it.
Also to reduce cost, he doesn't buy his sand, he uses some from a local beach. Now, because this sand isn't while, its hard to colour (so he doesn't), and his holds tend to be a blah brown.
Haven't tried this stuff myself, but if you go onto a site like Rockclimbing.com you'll find quite a few threads on the subject.
Rebecca