Because I have had some various difficulties with my PT SL, I have begun to do this before every ride. I called CycleOps customer service, and they suggested the following, very similar to the previous poster. They've really tried to simplify this procedure as I guess they get a lot of calls on it as the manual is a bit vague. I imagine they will update this in the next version so it is less confusing. Make sure there is no force applied to the pedals through this entire procedure. The items in parantheses are my own commentary:
1. Hit "mode" button until WATTS line is selected
2. Hold down "select" for 10 seconds (you can actually stop when the word "WATTS" starts blinking) and then release.
3. Hold down "select" again for 10 seconds (the manual says until WATTS display reads 0) and then release.
4. Press the "select" button again, just down hold it down this time, and this should bring you back to your regular set up and you are ready to roll. (The word "WATTS" should no longer be blinking.)
Hope this helps,
Ethan