Here are some tips to help you keep the rubber side down when riding in the rain.
As You Ride
• Brake early before a turn as brake pads work more slowly on wet rims.
• Take corners slower than normal. Avoid leaning your bike to turn as that can cause your tires to lose traction and slip. Keep your bike upright and steer it around the corner instead.
• Be careful about standing up and cranking on the pedals as this can cause your rear wheel to lose traction and slide.
• Use a thicker chain lube.
• Tape a baggie over your computer.
• Don't ever ride through a puddle in the rain, especially on unfamiliar roads. There could be a pothole.
• Painted lines on the road will be slick.
After the Ride
Mark the seat post height and remove it from the frame. Turn your bike upside-down. Do this somewhere where a quart of yucky water on the floor won't be a problem (that's how much could be in the frame). Hang the bike up to dry and rotate to a new position after a few hours to make sure you get all the water out. Re-grease the seat post before putting it back in the frame after it dries.
If you have clinchers, deflate the tire, hold the wheel almost horizontal and push the tire bead away from the rim at the point closest to the ground. More water than you can believe is in a rim will come out.
Don't forget to re-lube every moving part. It will save you hours of work later.