In all seriousness, I find a sweat wicking helmet liner to be a GREAT purchase. They cost like $10 - $20. They absorb the sweat so it doesn't drip down out of the helmet, then as long as your helmet has good airflow the liner dries quickly. That keeps you feeling dry and it does a great job of keeping you cool. (This can cause faster dehydration though so stay hydrated)

I don't see them in bike shops much. I see liners but they tend to be just material to keep the helmet from chaffing. Real sweat wicking head covers are better. Under Armour makes them, so do a thousand other companies. Check the football section of your local sporting goods store as they are common for football players.