Relax, wet weather can give you the best riding of your life. Be prepared to slide around a bit, the tenser you are the more you will slip around and the more difficulty you'll have saving it, remember your bike tend to save themselves if you just relax and let them. Remember the leaves on pavement + water + slip and slide. Put the frame sliders on, take the fairing off, and celebrate.
Seriously, staying warm is a key, the colder I get the stiffer I get..... rediscover the rear braake for some seriously fun slid-o-rama.
I have found any full coverage helmet will work well in the rain if you have a windshield. You can open the visor to the first lock and that will keep it clear.
A good rain suit, wetweather gloves and boots will keep you dry and comfortable.
The most important thing is good tires. If you want to keep driving when the rain comes down you need tires that will shed the water. If the rain grooves on your tires are too shallow you won't be able to keep up any speed safely.
I once road from Vancouver to Calgary in the rain and could keep to the speed limit easily because of good tires and smooth driving
I found out the Fog City thingy worked pretty well, but.. now my spectacles take the heat (cold!). So I removed the Fog city shield and drilled 3 small holes in the visor, hoping this will keep it clear. IMO a bad view is the worst problem in the cold and wet. A smooth ride will solve the other problems.
1. RainX - apply to outside of your helmet's windshield. This will allow the rain to bead off more cleanly. It greatly improves your visibility.
2. Fog City - This is a thin layer of lastic placed inside of your helmet's windshield. It keeps the screen from fogging up. An alternative option is to cover your nose and lower face off from the windscreen with duct tape. It's not particularly comfortable...but it also works well. (Though too many tape changes in a day will leave you looking like Rudolph the Red Nosed Reindeer.)
3. Rain Suit - There is nothing more uncomfortable than riding wet. There are good ones out there.
4. Water beading gunk - You can find many different types of water repellant chemicals to apply to gloves & boots. Some work some don't. But they should all help to some degree.
5. Tires - Tires don't heat up well when wet. They will be slippery. Add to that the fact that the first 1/2 an hour of any light rain the oil on the roads is brought to the surface....means you need to be very careful. Brake with both front and rear brakes...slowly...gently....and consistently. Avoid sudden moves if possible.
6. Beware of other drivers reduced visibility. If car drivers don't see you when it's clear out....then it's not going to get any better when they are distracted by poorer visibility.
Definitely wear a clear shield if you got one. I usually carry mine if there is even a threat of rain. Also crack the shield open a tad if it is fogging. My Shoei RF-800 has a little tab on the left side near the mount, which can be pushed. That opens the shield just a tad (less than a full click, like 1/2 that). This helps reduce the fogging. But be careful not to let the wind get in there and open it up.
Another tip is turning your head periodically to blow the rain off. This is easier than trying to wipe it off frequently, which just gets your gloves wet.
One post had it dead on, gloves and boots can be the most important. (Well if you wear leather as opposed to cordura suit, you really need a rain suit, because wet leather sucks.) I still don't have the gloves down right, although a winter pair I have does have a plastic lining to keep the inside dry. But I did recently bUy some riding boots (Alpinestars GPS) that make a big difference over the hiking boots I wore before.
The first thing I do when preparing to ride in the rain is put on my high viz yellow lime Aerostich suit and my Sidi on road boots. Both are lined with Goretex and extremely waterproof. I also clean the inside of my visor with Windex, which sort of helps with the fog. The basic gist of what I'm saying is wear the right gear, because it's dangerous enough out there when it's dry, let alone wet. And don't say anything about me being a rich bastard, I bought all this stuff with plastic. I couldn't afford not to.
The next thing I do before I start my BMW GS 1150 is turn the ABS on. Actually, it's always on, so I don't turn it off, like I usually do when it's dry. It works, as I have found out several times during heavy braking in the rain. Besides that, I definitely keep the shenanigans to a minimum, paying more attention to making sure car drivers can see me. OK, I will powerslide around a corner or two.
With the right gear, rain never has to be a reason not to ride.
As far at the visor fogging up... NOTHING beats a Fog City shield! They run about $12-16, and they're worth their weight in gold. It's just a thin plastic oval that adhesive's into the inside of your visor, and it completely stops fogging inside. Completely.
I'd recommend reading Sportbiking: The Real World (The Advanced Riders Handbook) by Gary S. Jaehne, the second part of which is dedicated to improving your wet weather riding (style). Before reading this book I hated riding in the rain, now I (sometimes) love it, but always feel relaxed and much safer out there!