I've been riding in the rain since... 1978 ? But I'm still here to tell you about it, so it can't be all that bad...

My tips and observations :
1) You can see through a rainy helmet visor much better than through a windshield with no wipers, so it ain't all that bad;
2) Like Serowbot said, lift the visor that inch (or half inch) you need to get a little circulation of air to prevent misting;
3) Watch out for stupid drivers who drive in misted up cars (many will keep the ventilaiton vents shut and just wipe the screen with their hand

)
4) Lights on all the time, when there's too much mist raised by trucks etc, flick the left indicator (cars will take it as the left indicator of a car and avoid pushing into your right side)
5) If it rains a lot where you live, get used to it ! BUT, on the bright side, with A/T or rain tires rather than the typical semi-slick cruiser tires, you will still have excellent traction

An example by Pirelli
- - - - - - - - - - A/T MC tire - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - MT66 (my ride) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Pirelli Supersports Semi-slicks

6) In a neighborhood, don't be shy and waste that horn ! Honk at anything, regardless, just to let then know you're there (many pedestrians will look down when walking in the rain)
7) If you have a good quality jacket, fine; now buy a pair of rain chaps and Totes, roll and tuck them in your tool roll (you DO have a tool roll, I presume?)
8) GLOVES ! When the weather looks like it'll rain, use waterproof riding gloves, or gloves with a woolen lining, to keep your fingers warm and dry.