Rain-X works, but i find it to be a little greasy for my tests. A nice coat of Z-2 (if your a zaino user) or klasse SG works really great on the windows. Same effect as Rain-X but lasts longer and gives the windows a nice shine.
Another trick, if you have a clay bar handy use that on your windows. Not quite as big of a difference as using it on your paint, but still really good results.