Properly wearing a properly fitting bicycle helmet helps keep your brain carrier (head, for those who prefer the monosyllabic) both safer and more comfortable. The least expensive helmets come in "Universal Size." That label is slightly oxymoronic, but describes a large helmet shell with a very adjustable retension strap system inside. For those with small heads though, the odds of looking like you're wearing a mushroom cap are quite high. Spending just a bit more will get you a helmet that's available in a variety of molded shell sizes (i.e. better fitting), that is smaller shells for smaller heads; larger shells for larger heads. Besides being more fashionable and a lot less mushroomy, since they fit better they offer a greater degree of safety and more comfort too. An additional point is that more expensive helmets (road and MTB at least) tend to have larger and more numerous vents which means your head will be cooler. They also weigh less which means your neck will be less tired/sore after a long ride with friends. BTW – All bicycle helmets have to pass the same product safety test, so heavier doesn't mean safer.
Have a friend or your friendly Bicycle Garage Indy salesperson measure your head circumference on a level line about 3/4 inch (1 1/2 – 2 fingers wide) above your brow. That number (in centimeters or inches) should fall within the head size range shown on the helmet box. That will tell you the helmet size you need from that manufacturer. Note that since there's sometimes overlap you might need different sizes from different manufacturers. The next step is to determine if you have a round or oval shape head (when viewed from above). Putting a roundish molded helmet on a more oval shaped head will usually create pressure points at the front and rear of the head. An oval shaped helmet on a round head usually creates pressure points on the sides of the head. The goal is of course, no pressure points on the head. For examples, historically Bell helmets have tended to be rounder and Giro helmets more oval. After Bell purchased Giro a few years ago the shape differences have seemed to be less extreme.
Proper wearing of a typical bicycle helmet means keeping it level so your field of view is not limited by the helmet or visor. Note how the racer, above left, has clearance above the brow line so seeing the course is easy. Note how the racer, above right, has a helmet possibly one size too large ("mushroom head") and seems to be wearing it tilted so that her view of the course is limited by the helmet.
A final note – 99.9% of the time, mountain bike helmets (i.e. with a visor) are not for use on road bikes. The reason is quite simple: the typically more aggressive road bicycle riding position rotates the head down so that the MTB helmet visor forces you to crane your neck back to see where you're going. Craned neck = sore neck. That 0.1% are cyclo-tourists who have their road bars adjusted very high in order to enjoy the scenery. Since they're sitting almost as upright as a MTB rider, the visor is usually no problem.
Proper use of your helmet is just one small aspect of riding your bicycle comfortably and safely. The major parts are getting the optimal size frame and getting it optimally adjusted to your body – that's where a BGI Bicycle Fitting comes in. Thanks for reading.
Thanks to Dangerous Girl in Safety Town for the "wearing a mushroom cap" image. Photos of racers (C) 2014 Frank Radaker.