I found this method worked great for fitting pants over my big behind. It is easy and I expect would work for many other alterations.
The method is described on the Threads Magazine website at URL http://www.taunton.com/threads/pages/t00018.asp. Even after using the Palmer/Pletsch fitting for pants book I still had trouble getting enough extra fabric to cover my big rear. I used the method for increasing the length of pants back found in this article and it worked way better than just increasing the inseam, which the pattern and the Palmer/Pletsch book suggested.