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(this is specifically in response to CRT lenses, which are slightly more demanding, but it's a good, cheap solution)
I quote from Guy Kuo:
Dry air blown across the lens. If that isn't enough, William Phelps described his cleaning solution....
1 drop dishwashing detergent (not soap!)
(or Kodak Photoflo if you have it)
1/3 cup 99% Isopropyl alcohol
1 2/3 cup distilled water
It works very well indeed and does not harm the lens coatings. It's dirt cheap to make to boot. Cheap enough to use on your car windows. You won't believe what using an optical cleaner on a car window will achieve. Windex is residue city by comparison and should NOT be used on your projection lenses for fear of the ammonia dissolving the anti-reflection coating.
Don't get overzealous about cleaning. Every time you clean the lens, you will create some minor scratches. In other words, clean when cleaning does more good than harm.
The above solution can be misted on and wiped off with a soft lint-free cleaning cloth. Use each portion of the cloth only one pass. Start at the center and work outword. You will need to lightly buff until dry or else there will be visible residue.
Another cleaning product I very much like is Formula MC-1. I was turned onto this by Steve Smith. It leaves absolutely the cleanest lens surface I have ever seen. Use as a last step. Just a few drops do the job. The lens will appear to fog over as this dries. Buff the "fog" off and the lens will be cleaner than it ever has.
In either case, never never grind dirt back into the lens. Remove as much dust as possible with dry, compressed air. Very very lightly dust with a CLEAN soft brush. It is so easy to scratch the lens with a slightly soiled brush. Don't wipe a dirty lens while it is dry. Use a cleaning solution to loosen and lift the debris onto a clean soft lens cloth.
Be enthusiastic in throwing out multiple cleaning cloths. You're trying to preserve the lens, not the cloths.
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