1. It must fit without any pressure points. Different companies have different shapes. If you have a circular head, and the hlmet is an oal shape it will not fit well
2. It must be comfortable, if it is uncomfortable you will not wear it
3. Ensure it can be adjusted to fit securely without compromising 1 or 2
4. Go for ANSI or Snell certified helmets, the UK EN1078 is not worth the paper it is written upon
5. Try and avoid angles - sharp edges and vents can catch and either cause the helmet to come off, or stop it sliding in an impact - Rounder, Smoother, Safer
6. It need not be expensive, a Snell helmet can b bought for as little as £30