Helmets work by absorbing an impact that would otherwise be absorbed by your head. It is normal for a helmet to crack or break after a crash; this indicates that the force of the crash was indeed absorbed by the helmet. In more minor crashes, a helmet may only show superficial scratches. However, any helmet involved in any crash, no matter how severe, should be replaced, since the helmet could be stressed to the point of being ineffective in further crashes.
The helmet should fit comfortably on top of the head, and should cover both the forehead (the bottom front edge should come down almost to the eyebrows), and the back of the head (the bottom rear edge should come down past the tops of the ears).
Most modern helmets have a headband that fits over the rear half of the head. This headband has an adjustment mechanism at the back, commonly a dial or slider, that can be used to tighten or loosen the headband. Adjust this mechanism so that if you gently shake your head with the chin strap undone, the helmet will stay in place.
Chin Strap Adjustment
Chin straps connect to the helmet (on each side) at both a front and a rear point. From these points, the straps converge just below the ear, and the straps on each side of the helmet are connected by a plastic clip under the chin. The tightness of the chin strap can be adjusted at this clip; the strap should secure the helmet to the head without interfering with head movement or breathing.
Use and Care
Helmets should be worn whenever you ride. Never drop a helmet on the ground, as this can weaken it and make it less effective in a crash. Keep your helmet clean and free of dirt; most helmets can be washed with mild soap and water. Many helmets also feature removable padding, which should be cleaned periodically. When in doubt, always follow the manufacturer's instructions for use and cleaning of your helmet.