I am Crest White Strips' biggest advocate. If I had a dime for every time I recommended them I'd be as rich as Oprah. Seriously, I just love them so much. They're inexpensive (compared to going to the dentist), quick and painless. I have friends who get their teeth bleached from the dentist and 9 times out of 10 mine are still whiter than theirs. There is really no excuse these days to not have white teeth with so many options available. Nice, white teeth really make a positive difference in your appearance.
The directions say to wear them for half an hour or an hour a day, but my trick is to sleep with them on occasionally. That will make a bigger difference faster. And I definitely don't wear them twice a day or even daily. I just put them in whenever I feel like it - usually once every other month and that's enough to keep them white. If you decide to buy them make sure to get the 'Professional' or 'Advanced' kind. They are a little more expensive, but worth it.