Riding in the rain can be an absolute blast, but there are a few small things that make all the difference between fun and misery.
In addition to what's been said already, minimize your trail braking techniques. You can trail brake in the rain, but not much ... at all! I recommend easing off the brakes before you actually initiate your turn, unless you are an expert trail braker. And if you are an expert trail braker, just trail a lot less than normal.
NOTE: DO NOT TRAIL BRAKE IN THE RAIN ON ANYTHING BUT FULL RAIN TIRES Otherwise you're just playing Russian roulette.
I try to overemphasize my body position in the rain to keep the bike up on the fattest part of the tire when cornering. You want as much contact patch as possible in the rain.
Smooth, smooth, smooth. I can't tell you how important it is to be smooth in the rain. Your margin for error is much smaller, and your inputs to the bike have greater effect. So be smooth in the dry, but be like the finest silk in the rain.
Soften your suspension for the conditions. A good rain set up is a lot like a streetbike, plush and gooey on both ends. The less harshly you transfer weight under braking and acceleration the more compliant the bike will be in the rain.
I know a lot of people think they can ride in the rain on something other than full rain tires, and they can ... but it sucks. I can ride in the rain on slicks if I want, but it sucks even worse. The only people who say you don't need rain tires in the rain have never ridden on fresh rain tires. Fresh rain tires are next to godliness in the wet.
Don't run on somebody's used rain tires. It's just not worth it. They took them off for a reason, and they were probably too old to begin with, plus they most likely haven't been properly stored anyway. The thing about rain tires is they're even more delicate than regular race tires. Once they heat up and start to show wear they're cooking the stickiness right out of themselves. They may still channel water, but they won't grip the same way.
Used rain tires is the perfect "penny wise and pound foolish" example. Saving a hundred bucks or so and crashing your bike totally sucks. Take offs in general aren't such a good idea, but used rain tires are a really bad idea.
A dry, fresh set of gear makes a huge difference if you can swing it. At the very least, dry gloves for when the rain stops. The heater in your truck can do wonders for drying out your gear.
And of course, the warmer it is the funner it is. Being wet and cold is a major drag, but riding in a Summer rain on a hot day is more fun than you can believe.
All this talk of riding in the rain is making me want to do it, but it's like 38 degrees where I'm standing, and we haven't even had our first event yet!
Come on Sun!