From the National Highway Traffic Safety Administration
Every bike ride begins with putting on a helmet. However, it's equally important to ensure a proper fit, so your helmet can provide the best protection for you.
Size can vary between manufacturers. Follow the steps to fit a helmet properly. It may take time to ensure a proper helmet fit, but your life is worth it. It's usually easier to look in the mirror or have someone else adjust the straps. For the most comprehensive list of helmet sizes according to manufacturers, go to the Bicycle Helmet Safety Institute (BHSI) Website at http://www.bhsi.org/
From the League of American Bicyclists
Helmet fit
To find the right size helmet, put one on your head without fastening the straps.
The front of the helmet should be level and two finger widths above your eyebrows.
Shake your head from side to side: There should only be a little movement.
Helmet adjustment
The side straps should come to a point just below your ears-move the small tabs on the side of these straps up or down until they are a half-inch or less under your earlobe.
The chin strap should be about half an inch below your chin when your mouth is closed.
Wearing a bike helmet with loose straps is the same as not wearing a bike helmet at all.