Depends on your location. You could buy a helmet with lots of vents and deep flow channels and still overheat from the intensity of the sun/uv rays. Wearing a light colored skull cap or bandana could help reflect the sunlight or shield exposed parts of your head. Some have liners and/or sweatbands (some made from coolmax) to wick away sweat and aid the cooling effects. Motorcyclists been wearing them for years. You don't have to spend a lot either. Here's where I buy my skullcaps:
http://www.bandanas.net/skullbest.htm

A cap would work just as well and you can find those anywhere. Cut down the bill or buy short bill ones or cycling specific caps.

Would carry a bottle of plain water and pour some of it over your head every so often to cool off if you are on a long ride on a hot/sunny day with little to no wind regardless if you are wearing anything under your helmet or not. You see the pro's pour liquids on their head on a particularly hot day as well as regular motorcyclists/bikers out on the highway.

Hyperthermia is the main health concern and many don't know when to stop, get in some shade, re-hydrate, etc., before continuing their ride. If I'm down at the beach during a heat wave, I'll sometimes stop by the free outdoor showers, hit the button, and wash the sweat off my arms, legs, and face/head or just go all out and jump in there...lol...feel a lot better in any case and everything will be dry before I get home.