* Helmets must protect the head against impacts, so the primary concern is stability and close fit. Most helmets now have a thin hard shell that protects the foam against the sun and cuts. Good helmets also have a web molded into the foam that keeps the helmet from disintegrating on impact (I once had a bunch of styrofoam fragments held together by this web after a crash, it really works). There should be plenty of air holes and thin padding strips. Avoid helmets with padding strips in front, where they can get soaked with sweat and cause sweat to run into your eyes. Different vendors sell different shapes; if one doesn't fit well and needs the thickest set of pads to stay in place buy from another vendor. For example, Bell helmets seem to be wider than Giro helmets.
Always tighten the strap until you can just barely open your mouth all the way. It needs readjustment every once in a while. Helmets that slip due to loose straps are no protection! Make sure that you can't pull the helmet back when the strap is closed.
Some people attach mirrors to their helmets and swear they won't punch through their eyeballs in a crash. They don't work for me because I ride road bikes, and all I can see in the mirror is my shoulder. It may work better for more upright riding positions.
See also an article on "Glasses"
