If you're gonna break up with her, have some fun. First off, don't hold back about anything. If you think she's gaining weight, let her know. Second, talk her into having threesomes, and eventually massive orgies. Bring home other girls who are interested. I tried this once. I had a friend who was a stripper, so I got her and one of her friends to come over and try to get my now x-gf to have a 4some. She didn't go for it, but the two strippers went at it for a little while. We broke up that night. Third, cheat on her as often as possible. If she finds out and dumps you, it's that much easier. Last, go out with YOUR friends and get drunk every night. If you follow these four guidelines, you'll be happy and single in no time.
BTW - avoid the break-up conversation, although if it does come down to that, try something like: "It's not you... Well actually it is you. I just don't find you attractive... You kindof disgust me. Besides, you're just to fat."