use a spray meant for lense cleaning, otherwise you risk removing the anti-fog coating... Best I've found is the paintball mask cleaning stuff. The lenses are basically the same material, and its cheap. (cleaning pretty much every round in paintball!)
(should be able to find it locally at any sporting goods store that sells paintball. Even walmart most likely has it, depending on area.
If the lenses aren't too bad, I just use a little cat crap.
1) 2000 Arctic cat ZR 700, which now thinks it is a 2001 ZR 600 EFI APV w/136" ripsaw. (for sale!)
2) 1995 Yamaha VMAX 600, which now thinks it is a Polaris! (Polaris x10 suspension, Polaris P85 clutch) (for sale!)
3) 2004 Yamaha Warrior
4) 2006 Yamaha Nytro