I was told this tip by a make-up artist in Nashville, tried it myself and it works wonderfully. Her orthodontist says its the same as the $300 bleaching treatments.
Buy a small bottle of Gly-Oxide which can be found in the toothpaste section of Walgreens, CVS or Walmart. Buy a plastic guard for teeth at any athletic store (mine cost $1.19 at a local athletic store). Put about a quarter size amount of the Gly-Oxide in the tooth guard and put it in your mouth. Wait about 15-20 minutes (you can put on your make-up while you wait) and in one application you see the difference. The lowers are a little more difficult as it takes turning your head upside down a bit, but as long as the Gly-Oxide gets on there, it will work. After a week, you will be amazed!