Avoid acidic foods
First off, stop using the whitestrips.
Secondly, rinse with a Fluoride
rinse, such as act or fluorigard. You can rinse with this several times a day. Also, while your teeth are sensitive, avoid drinking/eating acidic foods, such as grapefruits, grapefruit juice, oranges, lemons, soda, such as mountain dew, etc. You can also brush with sensodyne.