Botox: Stories

I received my first botox treatment in Feb 08. I was told the effects should last 3 - 6 months. Almost exactly 3 months to the day, it had completely stopped working. I got another treatment, that lasted only 2.5 months, and then another lasted approximately 6 weeks. I have been told that with each treatment I could expect the results to last longer each time. I have a friend who now only needs the treatments once every 10 months, so I am really confused as to why the results are so short term for me. I was getting the injections in the upper and lower forehead and around the crows feet, at $800 per session. I am not in a position to spend that kind of money 4 times (or more) a year, so sadly I stopped the treatments and am absolutely miserable. I was wondering if anyone else has had the same short term results and if there could be a possible reason for it, such as diet or whatnot. I did try another treatment provider the third time, as I became suspicious that I may have been getting a diluted treatment from the first provider, and explained the the last Dr. the suspicions I had, just to let him know why I stopped using the first one, so I really think it must be me.