From an extremely clever bloke i know ive picked up tips over the years, a couple of other things to try are blowing the laser assembly out with a can of air duster (invertible stuff is best incase it should decide to spray the propellant out), and also he mentioned removing the laser assemly, putting it in a piece of foil with some isopropyl alcohol, wrapping it up and giving it a shame to clean it.

Apparently the mirror below the lense also gets dirty.

Me myself i clean lenses with a cotton bud and non smear glass cleaner