The shape of the helmet is very important. I stick to certain models of Arai helmets, because I have a "Long-Oval" shaped head. LS2 & Icon also make Long-Oval helmets.
From Revzilla's website, Arai's helmet shape & associated models are described below:
LONG OVAL (very long front-to-back, very narrow side-to-side). On this end of the scale, we have the Signet-Q model, offering additional forehead room for those who find that most helmets have pressure points in the forehead.
INTERMEDIATE OVAL (a little shorter front-to-back and a little wider side-to-side than the Long Oval). This next fit package comes in the Corsair V, XD-4, Vector and Vector 2 models, which begins the shift from oblong towards round.
INTERMEDIATE ROUND OVAL (even a little shorter front-to-back and a little wider side-to-side). This next fit package is represented by the RX-Q. While the "roundness" is not dramatic like an HJC or Nolan helmet it is a slightly more round and forgiving fit for more Touring applications and wider brows. The RX-Q is Arai's newest most comfortable shape which should fit even more of a range of riders more perfectly.