Good tyres and common sense. If riding in a group get mudguards with flaps, it is only polite. Clothing, well that is up to you, I tend to wear the same wet or dry, maybe a soft shell if there is a lot of rain and a wind, mostly to avoid wind chill.
Technique wise, it is mostly common sense, avoid grids and white lines, ease up a tad more on corners, pulse the breaks a few times to clear the rim of water before braking for real. Pulse occasionally throughout braking to dissipate water on the rim.