I usually weigh about 185. When I went to Scout Camp this summer I came home weighting 173 after hiking 100 miles for ten days.
I tried to keep the pounds off by exercising and eating less and better food. (I believe that's the real secret. Exercise helps but I just eat WAY TOO MUCH) But slowly the pounds came back, my pants got tighter and no longer did people say, "You look great! You've lost weight!"
I stumbled across this article the other day with 7 ways we gain weight. Very, very interesting.