First, spend an afternoon at your local dealer trying out different brands (at this point don't mind the different types of helmets, just try out the type of helmet that the brand is providing), even if it takes an hour or three. If there isn't a dealer with at least a few brands a trip would be worth it. Try out each brand for about 10-15 minutes, if its the wrong helmet shape for your head you'll know, primarily at your forehead.
Second (the helmet shape that the brand is providing suits you), what type of riding do you prefer? Street, track, touring? Pick the right type of helmet for your, try different sizes (10-15 minutes per size and helmet). If you can get the helmet off your head with the buckle down (even if it feels right) it's the wrong helmet size for your - don't want it falling off when it's crucial. Together with the other guys before me, if it's primarily street riding go for something with high visibility.
Thirdly (the helmet shape, helmet type and size fits), Google the brand and see what they offer, is there perhaps a new helmet/evolution happening? Can you order the same helmet cheaper online? Research! The last step I guess is to ordering it.
Good luck!