If you're at all serious about bike racing, one day you'll have to race in the rain. And the best way to get comfortable racing in the rain is by training in the rain. Many racers I know don't like racing in the rain for one reason or another, but generally it's for mental reasons, and generally that reason is fear. Increased risk of slipping, lowered visibility, and a nervous peloton is enough to make anyone a bit nervous.
Not me though. I believe rain doesn't make things more dangerous, and I know that I can train and race in the rain, no problem. I've done it many times before, through heavy rains, through cold rains, and I know I'll do it again.
I actually believe rain lowers the risk of crashing in a peloton because racers take less risks in the turns, and generally pay a little more attention to what they're doing. It also makes less experienced racers stay home (who are usually the more jittery ones rain or shine). So if I see rain in the forecast, I usually get happy - a more select field, and many racers will be already mentally beaten a bit.
Like most things, I believe after having the right attitude about it, you have to practice. All that means is if it's raining and you have a workout planned, just go outside and do it. If you have a rain bike there's really no excuse (I have to admit, if it's a shorter workout I just don't want to have to clean my bike afterwards).
Here are some tricks for getting yourself out the door to train in the rain:
If it's raining out there, think of it as an opportunity to train in the rain. You can't control when the rain comes, so take advantage of it!
EMBRACE THE RAIN
Many people tighten up in the rain, trying to avoid getting wet, and then are miserable being wet. If you take a deep breath, relax, accept that you're going to be wet, and embrace the rain, you'll have a much better time. If you can do this it will help greatly in races, because no one can race as well when they're a bundle of nerves.
HTFU, don't cry about a little rain. Just get out there!