Check out reefcleaners.org - they'll set you up with the right amount for your size tank.
In my tank I have nassarius (big and small), conches, emerald crabs, skunk cleaner shrimp, banded trochus snails, astreae snails, margarita snails, nerite snails, cerith snails, and an urchin. I used to have 2 more urchins (great cleaners but were toting my corals around), a lettuce nudibranch (those things don't seem to last too long in our tanks), and turbo snails. Eventually I plan to add a brittlestar and an atlantic sea cucumber, but all in good time :)