As a professional in the Dental field, I recommend the in office whitening systems.  An appointment is scheduled(usually 15minutes) for the first visit.  Impressions are taken and the patient dismissed.  These impressions of the patients teeth are then poured into a stone model.  Once the stone impressions dry the trays are made.  These trays are very thin and trimmed to contour the gumline.  The patient is then scheduled to return to pick up the fitted tray and instructions given in the use of the whitening product.  The main component of the whitening gel is usually a 10-14 or 22% carbonide or hydrogen peroxide.   A B-B size drop of gel is placed in each tooth compartment and the tray inserted and worn for a minumum of 1 hour and maximum 2 hours at a time per day.  The majority of the brands have a 2 week supply.  Over the counter  systems work well but  tenacious stains require a professional concentration product that the Denal offices can offer.