I've used paper mache, porcelain clay (as in stoneware not the recipe burnley0 gave) and Sculpey/Fimo to make teeth. For me it depends on what materials I have on hand and what I want the final product to look like. With paper mache I can get giant odd-shaped monster teeth for really cheap, but it's one of the most complicated and time consuming processes. Porcelain clay is nice if you want something that looks really realistic and if you want something small and thin (fragile), but it's costly and requires a kiln to fire the pieces. Sculpey/Fimo is great if you want that clay look and only need a few small-ish teeth but you don't want to have to invest in a kiln, buy 50lbs of clay and spend the time drying and firing pieces.