What can bald guys do to prevent helmet sunburns? In the cooler months I normally wear a skullcap like this, which both keeps me warm and covers my scalp under my helmet.  

Now that summer is around the corner that skullcap is way too hot.  I'd like a way to protect my scalp without completely overheating... do any of my fellow bald guys have any tips?

Edit: To clarify, yes, I am wearing a helmet.  Any solution would have to be something that would work under the helmet.
Get a thin, tropical-weight wool cycling cap.  It'll keep the sun off your head but still breathe and wick sweat.  I have one from Walz that's been just wonderful.