Tomosy says to use a small amount of tissue, dampened with a lens cleaner (he's happy with Windex, I'm happy with Residual Oil Remover). Hold it at one end with forceps or hemostat, wipe only barely touching once across the lens with other end.
ANY pressure will remove the soft Leica coating on that lens. The tissue must be damp, touch it with dry tissue -- it will scratch. (Ask my Summitar how I know.)
The military Leica repair manual cites 180 proof alcohol as cleaning fluid.
If you can just blow the dust off, that's really the best solution.