I agree w/ everyone's posts so far...but will add a few things.
Even though you should stay off the roads for the first 20 minutes of a heavy downpour to wash away the oil, etc that accumulates on the road, sometimes you just can't avoid it. :?
So, as far as road position, where you should be riding when rain starts is in the tire tracks of the cars (lane 1 or 3 of the motorcycle lanes). Stay out of the center if you can because most likely that's where it will be the most slippery. Of course, if you have to move to the center to avoid a puddle (possible pothole) do so, then move back to the tire track.
Take your time when you're slowing down at off ramps etc, enter corners slower than you would on a sunny day.
Also, in case you haven't figured out a trick for getting the rain off of your visor, just turn your head from one side to the other for a few seconds, and the drops should slide off the side and help w/ your vision.
Gear is very important, because you will cool down much faster if you're wet, and shivering while on the bike is never good!!