Use the little micro-fiber cloths that eyeglass people give out/sell.

Just make sure it's suitable for cleaning optics. A fair bit of "micro
fiber" cloth out there is actually fairly ABRASIVE. Stick with stuff
designed for cleaning camera lenses and the like and you're much better off.
And whatever you do, do NOT go near it with any sort of "cleaner" that's not
specifically designed for the purpose. Many cleaners have solvents that
will make matters worse.