I use a product called "Bar Keepers Friend", found it in the grocery store for about a buck or so. Its a cleanser & polish. Comes in a container like scouring powder (Ajax, etc. ) gold color container, blue feild with white letters. I saw Sam use it on the old "Shade Tree Mechanic" show. It really works - you throughly wet your windshield, take an old wax applicator (sponge covered w/terrycloth) wet it, shake on enough of the product to make a pastey mix then in a circular motion apply to 1/4 of the windshield at a time (overlapping the sections) then rinse very throughly. If you wish to apply a glass wax or other product, dry windshield and allow to air dry all the way, then go for it.
It really works off the grime and dosen't scratch the glass at all. Be carefull around the black trim though.