I've actually been debating meal-replacement shakes on a fitness board. Sadly, too many fitness professionals would rather sell you some expensive product to make a quick buck than emphasize the need to eat a well-balanced diet. Most diet shakes won't fill you up as well as a meal that is high in fiber and protein.
If you don't like any jiggle in your butt, you can always wear compression shorts. They're difficult to get on and off, but they have some benefits.