This is super obvious to most people, but fenders keep you dry.

I've never been able to afford fenders for my bike before. Finally installed a set recently (SKS Chromoplastics), and today was the first serious rainy day I've commuted since. Normally I'll be soggy and striped by the end, but today I was only a little damp and COMPLETELY CLEAN. The price seemed high upfront, but they're worth every penny even after only one ride.