Agreed with the proper fit.
Be aware there are different standards for helmets. Most of the helmets sold around here meet Australian standards, made for motorbikes.
The better standard is a proper motorsport helmet. For some reason I believe they are illegal for the roads, but they are much stronger. They are tested with 3 or 4 different impact methods, instead of just one, and are also fireproof. The easiest way to tell them apart is motorsport helmets usually have a smaller visor gap.