Raleighrider has it right. The first thing you need to do is figure out if you're wet because of the rain, or if you're wet because of your own sweat. If its the rain, then you've got a leak somewhere. Find it and patch it. If it's sweat, then you're wearing too much for the work you're doing. Getting a good wicking base layer would be a good step. Using rainwear that's vented will also improve your situation.
Personally, I wear a complete SmartWool base layer (shirt, tights, and socks) under J&G Cyclewear raingear. The smartwool wicks the sweat away from my skin pretty well, and the vented jacket and pants let me adjust my airflow to the outside temperature.