The helmet should be level. If the helmet is for a child, make sure your child likes it. Convincing children to wear a helmet is easier if they helped choose it. Once you have the helmet, getting your child to wear it may be a different challenge. Insist that children wear a helmet any time they ride a bike.
Accidents can happen on sidewalks or bike trails as well as streets.
Finally, if you ride a bicycle alone or with your child, always wear a helmet yourself. If your helmet is damaged in an accident or any other way, you should replace it, even if the damage isn't obvious.