[mst]

Here's what I have successfully been using for years:



First blow off any dust. You can either buy one of the expensive photo-dedicated blowers (Rocketblower and the like) or get one for a lot less money in the pharmacy. Ask for a large enema syringe (that's what the dictionary suggests, "Klistierspritze" in German) and make sure it has not been treated with talcum powder (usually not the case). Works also quite well to blow dust of a camera sensor (or put loads of dust on it, if treated with talcum).



Second step is to spray on a cleaning fluid. I use this one:



http://www.micro-tools.de/en/Lens-Tools-oxid/Optical-Cleaner-4oz.html



Wipe it off with a tissue like this one:



http://www.micro-tools.de/en/Applicators-Tissues/Kimwipes-Box.html



Make sure to use each tissue only once.



Depending on the amount and kind of dirt it may take a few repetitions of step 2, but so far I have successfully cleaned any lens and filter I received with this procedure.



-- Markus