The Butt Builder

Superset (alternate exercises without rest)
Squat - 10 sets, 10 reps
Leg Curls - 10 sets, 10 reps

Reverse Barbell Lunges - 4 sets, 8 - 10 reps
Dumbbell Step Ups - 3 sets, 12 reps
Leg Extensions 3 sets, to failure

Workout Notes:
Squats - Do back squats (try not use a Smith machine if you can use a squat rack or dumbbells) but there are several variations you can substitute to change things up.
Squat and Leg Curl Supersets - Rest 60 seconds between each squat and leg curl superset. For example, perform 10 reps of squats immediately followed by 10 reps on leg curls. Rest 60 seconds, then perform this superset again. By the time you finish your 20 sets of squats and leg curls your legs should be pretty fried. Finish off the workout with 10 last sets of lighter weight exercises to finish off the legs.
Keep the rest periods short and the weight as heavy as you can go without sacrificing form. Keep rest intervals at 60 seconds-2 minutes max.