Don't bother with the cheap chit. Get Crest Advanced Professional Effects. Everyone is different, but my teeth/gums were not sensitive, so I used them every day. If you do experience a bit of pain, skip a day and resume the day after. They work quite well; everyone I know recognized I had whiter teeth.
I used them once but didn't keep it up, I switched to some whitening toothpaste for sensitive teeth after I finished the recommended ''routine''. I also stay away from coffee and smoking and anything that stains my teeth and my teeth have stayed as white as when I first used those strips like 2 years ago.
I did however get this new whitening toothpaste a couple of days ago that apparently shows whiter teeth after a single brushing but haven't used it yet since my other toothpaste hasn't finished yet, excitement.
How did I survive apocalyptic fires?
I simply refused to feel the flames.
I am trying out this 'Rembrant' peroxide/fluoride toothpaste, it's for superior whitening and I can already see it working and it's only been about a week! You can buy it at Walmart. It's around $5-7. You should try it.