M.J.
Hi A.-
I like the crest whitestrips classic - they're cheaper than the newer ones, more comfortable, and I think they work better on me. I use them every 6-9 months, and usually just do once a day since I forget the second time, and that doesn't seem to matter. Just a precaution though: at least initially, especially if you do them twice a day, it can make your teeth sensitive. You can maybe get some Sensodyne or some other sensitive toothpaste to help with that, but it lasts usually just maybe the first 2-5 days, then goes back to normal. Good luck!