just make sure when you purchase fish then, that you buy fish from stores whose fish don't have ich, and then you should quarentine the fish, some people will do this for quite a while, I do it usually for two weeks. But really just as long as you need to make sure the new fish isn't hiding any suprises. then be careful when you introduce the new fish into the tank. also you gotta keep your water nice and clean. allot goes into preventing these things.
Over all it's not that hard to prevent considering the amount of energy required to deal with the problem if unfortunately it occurs.