I just got home in the rain. My real world advice is to always carry a handkerchief/doo-rag. If you are wearing an open face or half helmet, the rain really hurts at speed. I pulled over and covered from my nose down. Worked great!. I wear eyeglasses so I don't think the biker masks will work well for me. I think the bill/visor thing on my half helmet helped to keep some water off of my face as well. And like it was said earlier, it isn't as bad as riding in a car without wipers. My glasses got wet but never to the point that it was much of a hindrance.

1) Stop for about 10 minutes when the rain first hits (if you can)- this allows for some of the oil to wash away.                                          
2)Ride basically in the path left by the left haand tires of the cars, the center does get most of the oil and fuel leaking.
3) Rather than pushing the bars (that usually means you are ..well..shall we say nervous..loosen of the grip..like you should do when crossing over a grated bridge.  
4) Most obviously, slow down a bit, increase distance to the vehicle in front; don't stop at the Stopsign but a bit before, Idling cars will have left oil etc right at the sign.
5)  Carefull of painted street markers (like cross walks and as noted earlier, metal surface like man hole covers.
6)and if you wear glasses like I do, invent some miniwindshield washers please.