I got these white strips while on a trip to New York recently having read so much about them online.
I was slightly sceptical at first, and a bit worried too that they might hurt my sensitive teeth. However, I was very impressed with the results!

They come as a small thin strip stuck on a piece of clear plastic which you peel off and then stick to the surface of your teeth. There is one strip for the top and another for the bottom. You then leave them to work their magic for the specified time.

After using the first strip there wasn't really much difference to the whiteness of my teeth, but by the third or fourth use (you use them daily) I could really see a change, and family members even started to notice.
By the end of the course my teeth were very white, and I didn't notice any discomfort or tooth sensitivity at all, and my teeth are usually extremely sensitive and I have weak enamel too.

Overall I would definitley recommend these. The only minor downside is that they are quite hard to get hold of in the UK. If you can track them down though, they are well worth it!