My insurance required me to do a six month doctor supervised diet program before they would pay for the surgery. I decided that since I had to do the diet program anyway, I might as well really apply myself and see how much I could lose pre-op. I lost 99 pounds (from 397 to 298) by the day of surgery. It was the best thing that I could have done for myself. I redefined my relationship with food, purged myself of carbs, and got into a positive mindset regarding eating and drinking before I had to deal with the realities of the surgery and the subsequent recovery. And it made the stalls that come along post-op SO MUCH EASIER to handle emotionally. (When my 3 week stall hit -- and it hits everyone -- I had lost 120 pounds, not just 20 pounds, and that made dealing with the stall much, much easier.)
As of today, I have lost 193 pounds total. Without all my pre-op work, that total would only be 94 pounds. I am on the verge of my goal; without the pre-op work I did, I'd still have a long, long way to go.
Good luck!