For both my Gentex and moto shields, I use Plexus plastic polish with a small microfiber towel that I keep around just for that purpose (in other words, it's the one "rag" lying about that I won't use to wipe up coffee spills and such). When I don't have a can of Plexus available, Lemon Pledge works a treat as dillardrg says - with the added bonus of the scent masking the fact that my helmet liner likely needs a good wash.
Since aerosol cans have a distressing habit of inflating themselves in a somewhat dangerous fashion in heat, Pledge does make small self-sealing packets of wipes which are fantastic for keeping in a flight suit/helmet bag/moto tank bag.