I do as many have suggested already - let the rain hit my glasses/prescription sunglasses and hope my brain compensates for the mess on the lenses.

I haven't been commuting long enough on a bike to have experienced something I've definitely dealt with while skiing in the snow (or worse - skiing in the rain!): rain hitting glasses (or ski goggles) is more pleasant than snow/rain pelting you in your bare eyeballs.

But, I ski a lot faster than I can pedal a bike.