1) Mudguards, FTW.

2) Kevlar belted tires and "slime" tubes. Flats are more common in the rain, and much less pleasant to deal with when it's raining.

3) Wool cycling clothing. Wool keeps you warm even when wet.

4) Have a designated "rain bike" that won't get you upset when it comes home dripping and covered with mud, worms, etc. and you can leave the heavy slime tubes and mudguards in place waiting for inclement weather.

5) Internal gear hub. Why wear out your expensive derailleurs with all the grit that accumulates when riding in the rain?