One of the best things possible to clean glass is:
1/4 cup rubbing alcohol + 1 1/2 cups water.
Mix it together, put it in a spray bottle, and get to work. Works for ALL glass surfaces. If you use a soft rag or baby diaper on your glass, you won't have the spots. Preferably, do it right after you give your car a bath, but that isn't really too critical.
Note: Try to avoid getting it on the paint as best you can.
OH! OH! OH! One more thing. . .
to lessen the amount of spots, make sure your car is not HOT! Whether it be from sun or use. Heat --> more spots!