Wear a white well ventilated helmet as I do. Never a black or dark painted one.

I take a wet bandanna and wear it pirate style underneath the helmet.

+1 on some kind of cloth covering on the head. I have a cheap Bell helmet that I recently replaced with a slightly less cheap Giro helmet. More vents definitely help, but I don't think they're a complete solution. I have one of those stretchy tube things that you can either wear around your neck (to block the wind in cool weather) or over your head in the summer. Keeps the sun off your head (you can get sunburn through the vents of your helmet for a nice polka dot look on top) and you can douse it in water in summer.

I've been using a "Headsweats" do-rag (Classic Cycling Caps | Cycling | Headsweats) for years. It fits well under the helmet, does a good job of absorbing, diverting and evaporating sweat. It has the additional benefit (like any cap or liner) of blocking sunlight; I'm a little thin on top, and with the do-rag I don't get sunburned in stripes. :D
Steve

Do you really think that color makes a difference? You're wearing a layer of beer cooler foam between the shell and your head.

The original post #1 says 4-25-09. And now, we're getting responses?

Yeah, I saw that too but I just wanted to hear what people had to say.

Why did you open it?

It was reopened by a spam post which has since been deleted.

Maybe there is a new device, other than what's already tried. Most all of everything is about the bike and how to make it go faster. Then we get the rider and the devices that help in some way. There are mostly passive systems but yet we need to see active systems. It has to do with the adversaries to the ride: heat and fatigue.

Yes the foam get heat soaked in short order with dark colors.