I don't know enough about the soup diet to know whether its soup and soup alone. However, from my experience trying to slice a few pounds, I would suggest adding in some steamed rice. It's almost impossible to eat enough rice to actually put on weight, and you can mix with just a little of something flavorful to make it go a long way. I particularly like a good spicy tomato soup with rice mixed into it. It's healthy, filling, tasty, and cheap.

Good luck, and don't forget the pictures next year!