1) Don't go to a brick and mortar shop and try on their helmets, and then order the one you liked best on line. Bad juju.

2) The rounder ones with better occipital lobe coverage also provide less twisting leverage in a crash. The ones that have tail fins and are pointy in the back are better for impressing small boys, and offer less drag when riding in wind tunnels.

3) Many of the top o' the heap, lightest, vetilated-est, best fitting, must have helmets of five to seven years ago are still in their manufacturers' line ups as today's mid-range helmets.

4) If the sales person at your LBS tells you helmets must be replaced after X-years, demand to see a manufactured-on date and refuse to pay full price for any that are over two months old, then enjoy the story you get about helmets only aging/deteriorating after the consumer buys them. (Helmet do age. It's complicated.)

5) If you buy an expensive helmet and crash in it, you must go buy another helmet. Do not let the helmet's high replacement cost compromise you into continuing to wear it.

6) Bell introduced the amazing Metropolis helmet system a few years ago. There was a removable visor, attachable rear-view mirror, an effective winter insulation kit, a snap-on rain cover with reflective trim and light attachment points. Compared to the Captain Billy whiz-bang racer helmets, the Metropolis wasn't even very expensive. They were only manufactured briefly, they sat on the shelves unsold and most were closed out cheap over the next several years. Cyclists won't likely see a helmet manufacturer try that foolishness again.