Cleaning the screen of a MacBook Air?

Use a very soft (e.g., microfiber) cloth. Do not use liquid.

While others around here will strongly disagree, I've been using a very soft cloth lightly sprayed with Windex on flat screens (including all my Macs) for years with no ill effect. If you've got "goop" on the screen, the only way to loosen it is with liquid of some kind; a microfiber cloth will just drag the foreign matter around and scratch the surface.