Water and a clean T-shirt, never use cloth towels or anything paper because they can scratch.
To make cleaning easier I suggest a good wax, I use Rejex but any good wax is fine. Plexius is very popular but doesn't dissolve bugs as well as water and leaves a very slight film that attracts dust.
I specifically use Rejex on the interior windows because it's very difficult to polish the film left from Plexius, one good interior cleaning in the spring and a no touch the windows rule.
Tom-