Info from Snell, the helmet rating folks:
Size: Try on several different helmets before you purchase one. The best way to gauge comfort level and fit is through comparison. The helmet should fit comfortably all the way around your head. Additional pads can be used to make it snug.
Strap: Pay attention to the chin strap. Make sure that the chin strap fits around your ear and under your chin snugly and comfortably. The helmet should not shift on your head.
Straight: Know how a helmet should fit. A helmet is meant to be worn low on the forehead, just above your eyebrows. Look into a mirror or have a friend/parent help you determine the proper fit.
Sticker: Look for Snell certification. Snell Standards are the most stringent in the world, exceeding those set by the Consumer Products Safety Commission (CPSC), American Society of Testing Materials (ASTM) and the American National Standards Institute (ANSI). Need to know which helmet is Snell certified? Browse through our helmet certification lists.
http://www.smf.org/top4.html
