I've been reading up on this, since recently I'd done the same thing to my custom green.
I have a removable module on mine which makes it slightly easier, from seeking help from a few people over at photonlexicon, I'll give a a run down on some of the PM's:
First head for a pharmacy or chemists I belive you call them. Tell them what your doing and tell them the tool of choice is a wood sticked cotton tip swab. They should be able to sell you a sealed pack for about 2 euros.
Next you need some acetone, which can be used to gently remove the dust or crap on the lens, then you need some methanol which removes the residue acetone leaves.
Make sure you get very pure stuff, and not the cheap rubbish that is transferred in 55 gallon tubs.
The other option is:
Rob (Stanwax) over at photonlexicon.com, he sells some polymer lens cleaner. It's not cheap, but a small bottle will last forever! You just paint this on the offending optic, let it dry and then peel it off. It comes off the lens in a sort of stretchy sheet that contains all the dirt and dust that was on the lens. This leaves the lens pristinely clean
Well hope this helps.