I bought some Plexus plastic cleaner and protectant from JCWhitney. WOW, it works great. You spray it on and just buff it off with a soft cloth. I also used it on my entire helmet including the face shield. My helmet shines like a dime in a goats butt!!. You can also clean your golf cart windshield without scratching.

I've been using Walmart brand (I have no idea what's in the stuff, but they claim its safe for all lens)  lens cleaner on my helmet and visor. It does a great job of cleaning the bugs off, and doesn't leave a visible residue.