I use a paper towel folded up with alcohol. If you use a screen protector that is the best way. Windex and the likes can cause the screen protector to discolor.
I typically just use my shirt and a little fog from my breath (same way to clean glasses) but I got an undisclosed substance on it and so now I don't think that way will work, but from what I've seen a little bit of rubbing alcohol seems to do the trick