Nobody has suggested a simple head band (scroll to the bottom of the page). Mine is from Giro, but many others make them specifically for under the helmet.

I sweat like crazy and had the same problems as the OP. I used to just squeeze my helmet like oboeguy, but that just got disgusting and I had to stop all the time. It got really frustrating. I don't want to stop a training ride 10 times to wipe the sweat from my eyes and clear out my helmet.

These headbands are open on the top so you don't get too hot and do what they're supposed to, just keep the sweat from your eyes. They are thin enough that they fit inside the helmet so you don't look like a noob or a ****** trying to ride a bike.
f