RAIN-X to prevent water from building up in the first place...
 

A good time I have and use is to apply a coat of RAIN-X to the windows after washing and polishing them up. It usually helps the water bead up and slump right off. I put a new coat of RAIN-X on about every other wash.

TonyPTX

i was wondering about this, i bought a new car, and the guy before me did not take great care of the outside, and the car has t-tops and they have some very ugly water spots.