Lifting a saggy bum involves strengthening and toning the glute muscles. Start off by adding exercises such as glute bridges, hip thrusts, and squats to your routine. Include cardio exercises such as running or stair climbing to burn excess fat and reveal the muscle definition in the underbutt. Consistency in performing these exercises with proper form will help lift and firm the glutes over time. In addition, maintaining a healthy diet will also help reduce excess fat and firm the butt.