Like 'em or not, helmets are just a good idea. They are the only thing that is going to protect you from an impact to the old noggin, which is unfortunately more common than it should be (mostly due to poorly designed cities and drivers and pedestrians -- even if you are a great cyclist, it's other people you have to worry about).
Whether you are purchasing your first helmet, updating an old cracked or out-of-date model, or helping a child or friend get a helmet, knowing how to fit and wear a helmet correctly is important. If it's not on there right, it could do more harm than good -- seriously. That's why good fit is so important.
Fitting it Tight
Getting a good fit for a bike helmet is pretty easy, but you want to make sure to hit all the bases.
First, you want to measure the diameter of your head with a tape measure. Use this measurement to choose the correct size of helmet. When it's on your head, you will probably need to adjust the fit by turning a dial or adjusting straps at the rear of the helmet, or by adding or removing additional foam pads from the interior.
Wearng it Right
When the helmet is properly fitted, you should not be able to wobble it from side to side, and the front of the helmet should come within about an inch and a half of your eyebrows.
The buckle should be snug under the chin and adjusted so that the attachment point is centered under the chin. It should be snug enough that no more than one or two fingers can fit between it and the chin. The side straps should be adjusted to form a V-shape just underneath and in front of the ears -- this will provide maximum comfort and security.
If your helmet is adjusted properly and fitted to your head, it should shift downwards with your forehead when you yawn. If it doesn't, it might be best to try another model that may match the shape of your head better.