Stay off manhole covers and painted surfaces such as the center/shoulder lines.
What the driver sees.
You didn't say what you were riding but if it has ABS you don't have to worry about breaking in the rain as that is the big benefit of ABS. You just break like normal and let it deal with traction.
Now, traction while riding in the rain (non braking) is another issue and others have addressed those points well.
'05 1200GS Big Blue
...so what do I know about riding in the rain?
I've heard it said that wet roads decrease traction by approximately 30% - so as a result, I ride 30% more conservatively when the road is wet. 30% more following distance. 30% slower through curves. Start braking 30% sooner at stop signs/stop lights. You get the picture. Pretty soon it becomes second nature to simply take it easy in slippery conditions.
I am firmly of the belief that there is no such thing as bad weather - only bad gear.
If you can keep your vision clear, your body warm and dry (especially hands and feet), and others can see you, you've won the battle against the conditions.
I could type paragraphs on the gear I use if anyone is interested. Just let me know. Let me say that I ride the bike I ride because it has ABS brakes, which I consider essential equipment on the wet, slick and hilly roads I travel on a daily basis.
I've ridden with several people who become very nervous and anxious about riding in the rain شاِ even to the point of panic and freezing up. I think it might be worth pointing out that even though riding in the rain requires extra attention and a few special precautions, like the ones already mentioned, there's little reason for any fear about it if some common sense is used.
Really, it basically boils down to going slower, paying more attention and watching for rain-caused hazards like standing water. For those with excessive anxiety about riding in the rain, the pavement doesn't turn into an ice skating rink, the bike tires still have reasonably good traction, the brakes will still work and with some extra attention, a light rain can even be sort of fun. In other words, slow down, stay relaxed, pay attention, use common sense and enjoy the ride. The worst that's likely to happen is that you'll get a bit wet.