As many people have mentioned, the trick to buying a helmet is knowing the shape of your head versus the shape of the helmet. Most LBS can explain the differences between the helmets that they carry, one can't always assume that every model in particular brand all runs the same... quite often they don't. For those that have long/narrow shapes, Louis Garneau is a good brand to try.
Uvex helmets are great helmets, unless you have a small head. They make various sizes, but the outer part of the shell is only one size and has too much play if you're like me and have a small head.

The key here is to try EVERYTHING on before you buy, not only will you become knowledgable about what you like/dislike, but you'll find what fits and that it what is the most important!!