Originally Posted by phinney
Get one with a Snell sticker in it - they actually get tested. Given a choice opt for fewer ventilation holes, thicker shell, harder shell, and more head coverage. The good news is the most protective helmets also tend to be less expensive. Not sure what kind of riding you're doing but the BMX helmets offer much better protection than the roadie/mtb helmets.
Along those lines of a BMX helmet, there is Pro-Tec. They have a model called the "Ace Dig", its basically the normal BMX'er helmet, just add a visor.
Most important thing, find one that FITS. I know hat was said already, but it cannot be stressed enough.