Crest has been coming up with a great line of whitening strips ranging from basic white to obsessedly bright. I tried them exactly by the book for awhile and enjoyed the results until i began getting unwanted results. I know these now, but I've noticed a lot of my friends aren't entirely aware:
Regular usage breaks down tooth enamel making teeth extremely sensitive and almost see-through.
Stop after near desired effect to not look like a cartoon character.
Take your time. The best way to use these products are every other day to a couple times a week to avoid negative reactions.
Don't overdo too many whitening products.
I'll still use these products but I've found that wanted results takes time.