At the Earl's Court Broadcast Video Expo last week, I came across a firm (True Lens Services) who specialise in repair and maintenance of lenses. In an FAQ on a handout, one question was of course "How do I clean the glass?" Answer: "Better still, don't get it dirty in the first place! But if you do, start by using a puffer bulb to blow away as much debris as possible without touching the glass. Then gently use a camel-hair brush, starting in the centre moving in circles towards the outer edge. When there is no evidence of dust or dirt, use a lens cleaning tissue with an appropriate cleaning chemical."

I'd add to that a) as has already been said, a lot of surface debris can accumulate before becoming a problem, and usually then only when shooting into the light. b) when brushing the lens, hold it upside down so that debris falls away from the surface rather than just being distributed over it. c) lens tissues have never impressed me - they are usually too hard and crinkly to follow the surface of the lens without excessive pressure. I prefer Tomosy's approach of using a plain soft, facial tissue, with just a bit of heavy breathing (!) on the lens unless any grease was involved.