Tip one: Keep the rubber on the road. failure to do this...sucks.
Tip two: It only rains in the front, but time will allow it to circle around and join at your back. Wear appropriate riding gear for the occasion.
Tip three: Avoid crosswalks and other painted surfaces, this goes back to tip one.
Tip four: Riding in the rain should be avoided if the weather is not warm. Cold and rain suck.