1. Getting a tape measure to check the diameter of your head is a waste of time, don't bother. We all have different shaped heads. In the bike shop I found it is not just the size of the helmet that makes a good fit, it is often the shape of the helmet shell and whether that shape is compatible with your noggin.
Try the helmet on and tighten the adjuster mechanism, (usually located at the back of the helmet), until it is firm but comfortable; remember you could be wearing it for hours at a time.
2. With the straps undone, check the position of the helmet on your head. You want to make sure the helmet is not tilted too far back, exposing your forehead, nor too far forward, exposing your occipital bone (back of skull).
The 'two finger' method is one way of checking this. Simply position the helmet so you can fit two fingers between your eyebrow and the helmet.
3. With the straps still undone, move your head around vigorously and even bend forward tipping your head upside down. This simulates riding over rough terrain, and if fitted well the helmet should stay in place.
Hopefully this isn't a position you will experience while riding, but the exercise is to check that the helmet will remain in the correct position in any circumstance.
4. Okay, the helmet should now be fitted comfortably and positioned correctly. Now for the straps.
It is important to know that the most crucial fitting is the helmet shell; the straps are secondary to this. They will keep the helmet on your head if you take a fall, but should not be used to compensate for an ill-fitting helmet.
Adjust the straps so the "V" is sitting just below your ears and do up the buckle. Tighten till the point that you can still fit a couple of fingers between the strap and your chin.
5. Check everything is in place, that the shell is positioned correctly and that you are comfortable.
Go ride a bike!
Currently only Australia, New Zealand and a couple of states in the USA have mandatory laws regarding wearing bike helmets. I know here in Australia you also get a hefty on-the-spot fine. Good for business in the shop (we sold HEAPS of helmets), but annoying when you just want to pop to the shop for milk and don't want to fuss over a helmet.
The police once staked out the local high school at last bell and nicked every kid without a helmet (had to order more helmets after that one)!
Most U.S States and municipalities have no laws or regulations regarding helmet use. 29 states have no statewide law and 13 of these have no laws in any jurisdiction either.
On the map states with statewide laws are BLUE, states with local laws only are RED, and states with no laws at all, GREY.
There has been a lot written about the effectiveness of mandatory helmet use in reducing death or major injury and the jury is still definately out. Medical and road safety organisations frequently support the creation of more laws for helmet use.
One study in Australia by the University of New South Wales concluded that mandatory helmet laws resulted in a 29% reduction in head injuries, while a similar study in New Zealand showed very little or no reduction.
Personally I think we should have choice. Keen to hear your thoughts on the matter in the comments below..!