Wear waterproofs!! And as mentioned above, invest in the rain gloves, they really do work and get better as they get wetter.

I don't bother with an umbrella, they just get in the way IMO. An extra hat or two doesn't hurt.

Assuming we're talking about RAIN and not drizzle, you'll probably find that your approach shots stick to the green a bit more, also the greens will break a bit less than when they are dry, so you can hit firmer putts.

And always remember that even though it is raining, it beats being in the office.

I hit my equal best tournament score in very rainy conditions - the rain helped keep me focussed on each shot.