Correct Helmet Fit
The most important piece of bike gear you will buy is your helmet, and the most important piece of your anatomy is your brain, so you need to get it right.
The cost of a helmet is not the issue here - all helmets have to conform to certain standards, so although a more expensive helmet may offer a little more protection, even the cheapest ones have to protect you well. The difference in price is usually more down to quality of things like the interior and visor fittings, than the quality of the actual shell.
Firstly, make sure you get the right make. Not everybody's heads are the same shape, and not all helmets are the same shape either. I for one, much as I would like to, cannot wear Arai helmets.
When trying a helmet, make sure that any pressure is evenly distributed around your cranium. If there is say, undue pressure on your temples or forehead, try a different make.
Although helmets may carry the same size markings, fit can differ from make to make, and even between models from one manufacturer
When you try a helmet for size, it should err a fraction on the tight side, to compensate for compression of the interior padding. This compression is necessary to ensure that the padding moulds to the shape of your head, and thus remains securely in one position. However, as said above, there should be no undue pressure at any point on your skull.
When you think you have the right size and fit, put the helmet on and fasten the strap securely. Get a friend to stand facing you. Lower your head, and get your friend to grasp the base of the helmet at the nape of the neck and try and pull it forwards over, and off your head. If there is undue movement, try the next size down. If you get to the point where the helmet is tight, but still wants to come off, then try a different model/make. The helmet should fit properly WITHOUT the strap being done up, only do up the strap before attempting the lift over the head backwards test
Another test you can do is to get someone to hold the helmet and you to turn your head from side to side, as if looking left and right.If there's any movement the you should really try another size. The padding on the cheeks should be as tight as possible IMHO, and then it will give a perfect fit when bedded in. If you feel like you have hamster cheeks then this is a Very Good Thing.
Don't ever buy a helmet mail order - because helmets are generally hand-assembled, there can even be slight differences in fit between two helmets of identical type.
What price your life? - get it right first time
Always be yourself. Unless you can be a unicorn. Then be a unicorn