Whether you're after faster sprints or a better butt, the hyperextension machine is a must. Start with bodyweight only and really think about making the glutes, not the hamstrings, do all the work. Squeeze them hard at the top.
To progress it, add bands. Begin bent over a hyper machine with the band held around the back of the neck. Extend to a straight position and hold for 3 seconds. Lower under control and repeat for 3 sets of 6 reps. No band? Hold a plate or dumbbells.