Toothpaste: the answer to whitening-related sensitivity
Have the office apply topical fluoride after whitening. Use a system that has been proven to minimize sensitivity or prescribe fluoride toothpaste for after treatment use. If you have the whitening trays, then put the prescription toothpaste in the trays.
Also, before treatment, use a desensitizing toothpaste (one that contains 5% Potassium Nitrate) twice daily for two weeks prior to whitening (if you are using trays) and for two weeks there after.