The most important thing you want to realize when you're riding your bike in the rain, is that the bike is going to handle a little bit differently. The brakes aren't going to work just as well. The tires aren't going to grip just as well.
You can actually adapt to these situations. The first thing I do if I'm about to race or go on a long ride, and it's raining out, is I lower my tire pressure. What I do is I take my tire, open up the valve, I'm usually running about 110 p.s.i., and I want to lower it down to about 90 or 85, test the tire. That about 3 seconds drops it down to about 90 or so. I close it up, and I do it to the opposite, as well.
The reason I do this is that I get far greater grip. I get better traction. I can corner better. It's far less likely that I'm going to slide out in a turn due to wet weather.
I like to wear sunglasses all the time, but I find that in the rain, it helps even more, it's even better if you put in clear lenses. It's a great idea to have a set of sunglasses that have interchangeable lenses. You can choose clear lenses for riding in the dark or riding in the rain, then you can choose darker lenses for the mid day, and medium lenses for sunset and sunrise.
Another thing you're going to notice when riding in the rain is that parts of the road are going to be a lot slicker than others. The reflective lines and the reflective strips are going to be far, far slicker, avoid those as much as possible. You want to stay on the pavement.Make sure you don't turn on any of those.
Also, make sure you don't turn on any of the manhole covers, or any sort of steel that's on the road. It becomes incredibly slick, and you're going to find yourself sliding out.
Wet roads also add to the likelihood of flatting, you're going to find there's more debris on the road. You're also going to find that your tires are going to be picking up far more glass, far more objects that are going to puncture your tires. Make sure that you're prepared for riding in the rain. Expect to get a flat.
And that's what you need to know before riding in the rain.