It is important that your helmet should fit correctly. Most manufacturers provide a range of sizes from children's to adult helmets, with additional variations of small, medium and large. Helmets are held on the head with nylon straps, which must be adjusted to fit the individual.
Most helmets will have multiple adjustment points on the strap to allow both strap and helmet to be correctly positioned. Additionally some helmets have adjustable cradles which fit the helmet to the occipital region of the skull. These provide no protection, only aid a better fit.
In general, the more skull coverage a helmet provides, the more effectively it can be fitted to the head and hence the better it will remain on the head in the event of an accident.
The helmet should sit level on the cyclist's head with only a couple of finger widths between eyebrow and the helmet brim. The strap should sit at the back of the lower jaw, against the throat, and be sufficiently tight that the helmet does not move on the head. It should not be possible to insert more than one finger's thickness between the strap and the throat.
Any helmet that has sustained a substantial blow should be discarded and replaced, including any helmet involved in a crash in which the head has hit a hard surface or in which a fall has resulted in marks on the shell. Because some helmet materials deteriorate with age it is recommended that your helmet be replaced at least every 5 years, sooner if the helmet manufacturer recommends it.