I started whitening my teeth early this year after getting my hands on Crest 3D Whitening Strips. I've heard of a lot of good reviews online, so I bought them on eBay (as you can't currently buy them in the UK, boo!). I've tried lots of whitening kits from Pharmacies and supermarkets, but none of them match up to the results I get from American brands. Bearing in mind I am not a professional, these are only tips on how I like to whiten my teeth (remember to ask your dentist before buying whitening products).
Try not to brush your teeth with toothpaste before applying the strips. The fluoride from the toothpaste can interact with the chemicals in the strips, thus making it not as effective. Instead, brush your teeth with toothpaste a few hours in advance and then brush them with only a wet toothbrush half an hour before applying. I will brush my teeth after dinner, then again without toothpaste before I apply the strips. I find brushing before opens up my teeth's pores, allowing the whitening to be more effective!
The Crest strips I use have advanced seal technology, which allows them to stick securely to your teeth without sliding around during the process. However, if you're using another brand, you might find they won't stick onto your teeth as well. A trick I've found helpful, is using my dental retainers (the clear ones you are given after braces) to keep the strips in place. It actually works pretty well, considering the strips are fairly thin, the retainers fit comfortably on top.
If you're finding that the strips hurt your teeth after use, try using them every other day instead of daily. I have sensitive teeth, but I find if I have a day off whitening, my teeth are fine to whiten again two days after. Don't worry about the whitening process, it doesn't really matter if you don't use the strips daily like advised. It won't make a difference to how white your teeth will be at the end.
After use, instead of using a toothbrush to get rid of the residue, try only washing your mouth out with water. Brushing your teeth around the period you whiten your teeth can make them very sensitive and brushing off the residue can reduce the effects of the strips. I like to apply the strips before bed so I can gargle water afterwards and sleep to let the chemicals do their magic.
Have you whitened your teeth before? What are your tips?