Vanity
Virginia Postrel points to something that would keep me going to the Dentist regulary: Free Whiting:

When I visited the dentist yesterday, they gave me a new incentive to keep up my regular checkups (as though all the dental work I suffered through after not having regular checkups weren't enough): free whitening for life. This approach suggests a whole new approach to promoting preventive health checks. Since their behavior suggests that most people care more about their immediate appearance than their long-term health, maybe doctors should offer discounts on Botox (or Viagra?) to get patients into the exam room.

I just went through a box of Crest Whitestrips which are a bit of pain to do for 2 weeks, but better than forking over $400 for the Dentist to do it. It would be nice if Dentist could kick that in for free.