LifeHacker has the right approach with their UltraNewb guide to the upgrade. Six steps and you’re done.
For our purposes, we’re going to assume your upgrade path is a straight Leopard to Snow Leopard upgrade, though a clean install is always nice if you really want to get that fresh start feeling.
For the first time ever, I did both a clean installation and Apple’s recommended upgrade procedure. Both installations were flawless.