Windex*...and yeah I know it's "bad" for it but so far I have no ill effects and its been about 18 months...Oh and I rain-ex* it and my helmet shield too...with some crappy old rain-ex* from a million years ago and no fogging either....perhaps I'm just lucky...nah I would have hit Lotto by now if that were true...
* DO NOT use these products on YOUR bike it WILL screw them up! The author is just having a run of Sh!tluck soon his Mephis Shades shield will be a giant ball of scratched (he uses paper towels with the windex) foggy yellow junk.