The first option, as stated by Gabriel is to blow it dry. I prefer the Giotto Rocket Blower. It is powerful and will not spit goo on the lens like some canned air can do.The next step on the ladder of dirtyness is a soft lensbrush. If that fails simply use a cloth, microfiber or otherwise.The thing here is not to push any sand around.I've never harmed any lens by cleaning it with a soft clean cloth (at least not for the last 50 years), if necessary (fingerprints!) moistened with some medicinal alcohol or Eclipse fluid. The thing is not to wet it through and through with the fluid, as the kit between the lens elements may dissolve, especialliy with older lenses. And NEVER use optical cleaning fluid meant for glasses etc. Those may contain chemicals that will harm the coating. Reading this back it seems like some complicated recipe. It isn't; in practice, in haste I've been known to use the tail of my T-shirt after breathing on the lens with no harmful result.....