If you haven't ridden in the rain before, pull your breaks somewhat gently a few times (letting go afterward) before you actually plan to stop. The pads will "wipe" most of the water off your rim, restoring most of your stopping power. Kool Stop salmon pads have a bit more stopping power in the rain.

That's probably obvious, but after riding a CX bike with disc brakes for years, I had forgotten about rim braking, and was a bit surprised the first time I took my new road bike out in the rain.