Best advice: Use a high fluoride toothpaste before using white strips (like fluorodex) - maybe for a month. While using white strips, dont use them right after you brush - wait an hour or so. Don't brush right after using the strips. Drink LOTS of water! and use a toothpaste like sensodyne to help with the sensitivity.
Overuse of white strips can degrade your enamel. White strips are acidic and that is what causes the sensitivity. Overuse can erode your enamel!