To lift the butt try this one:
Get down on your hands and knees, with your hands directly under your shoulders and your knees directly under your hips. Lift one leg and straighten it out behind you at hip height. Lift and lower the leg, always trying to keep it at or above hip height.
There are lots of variations of this, you can do it with the knee bent so it's like you're trying to kick the ceiling or you can rotate the leg in circles... you can also do this lying on your stomach which is more difficult and works your lower back quite strongly as well as being good for your butt.
Sorry if those instructions aren't very clear.