Scattered clouds beget drops on your face shield, then a deluge of semi-Biblical proportions. Pavement that was dry and grippy 15 minutes ago is cheek-clenchingly shiny. What do you do? First, don't panic. To quote a Bene Gesserit incantation from Frank Herbert's Dune, "Fear is the mind-killer." Scared? Slow down. Find a bit of cover and pull over if you need to. Run through your options. Presented with enough rain to choke healthy adult toads, calculate the most direct route that puts a nice, dry roof over your head. Especially if whatever you're wearing isn't waterproof. Is your shield shedding water well enough? Is it fogging inside? You can't deal with what you can't see, so fix that first.
Since the 100 billion neurons between your ears are less efficient in cold/wet weather, rule one is slow down. Trying to get where you're going too quickly all but guarantees a stupid mistake. Don't rush. You don't have a lot of experience in 100 percent humidity? See rule two: relax. No sudden moves with the throttle, steering or brakes. Smoother is faster on any sort of pavement. In the wet, it's indispensable. Get a feel for available grip and adjust your lines through the corners accordingly. The straightest trajectory through a particular set of bends is usually best. Anything you can do to make the tires' job easier helps the cause. Don't lean over any farther than you have to. Riding in a taller gear helps the rear tire hold on without spinning. The engine should still respond on cue, so don't lug it, but an extra upshift can help soften power delivery just enough when traction is scarce. So does rolling on the throttle as gradually as you can and not accelerating any harder than you need to. The same goes for the brakes: ABS is a blessing on wet pavement, but whether you're so blessed or not, go easy on that lever. Since your right foot isn't as sensitive or precise as your right hand, go even easier on the pedal, especially if yours tends to be a bit lock-prone. And since you can't brake hard, brake early, staying tuned in to what the front tire is telling you about available grip.
How are you sitting on the bike? Cold muscles tighten up, so stay loose. Shift around on the seat when you can. Shrug your shoulders a few times. Move those arms a little when it's safe. Clench and relax the muscles in your back and thighs to keep the blood flowing. Keeping the balls of your feet on the pegs will give you a better feel for how the tires are doing down there. What else? Make sure you can still see well enough to read the road. Deal with anything that compromises clear vision immediately. Contact lenses beat glasses 10 times out of 10 on a rainy day. Stay away from those painted lines. Same goes for puddles: If you can't avoid them, slow down and proceed very carefully with no braking, steering or throttle inputs until you're on the other side. If the road is seriously mucked up, you can usually skirt some of the filth by riding in the tracks of cars that have gone before. Random crud? Riding nearer the center gives you room to avoid that stuff. And if something looks slippery, assume that it is. Once you learn to keep accurate tabs on what's happening inside and outside your helmet, riding in the rain is nothing to be afraid of.