Create a module for argument checking that all programmers can use when validating data coming into a method. This keeps a major source of redundant code in one place, and ensures that you know something about the safety of your code across the whole project. It also means that you can simply turn off argument checking if you want to for speed.
I, the omnipresent Anonymous Monk, wholeheartedly second this advice. It saves a lot of work and results in far simpler, more flexible software. It took me far too long to start doing this myself. Good stuff.