How to Exercise Buttocks While Sitting
If you want to tone or strengthen your buttocks while sitting, there are many simple exercises you can do. Try squeezing your gluteal muscles and holding them for 30 seconds. Do this for a few sets of 10 to 20 repetitions. You can also do squats while sitting down. Just plant your feet about shoulder-width apart, raise your buttocks an inch or 2 above your seat, and hold the position for as long as you can. Repeat this throughout the day to strengthen your buttocks. If you're at home, another thing you can do is replace your chair with a stability ball for an hour or 2 a day. This makes you use your leg and buttock muscles to stay balanced. You can also bounce slightly while sitting down to work yourself harder.
Squeeze your gluteal muscles. The easiest way to work out your buttocks while you sit is to simply squeeze the muscles in your rear end. As you squeeze your gluteal muscles, you will feel your rear end squeezing in and your hips will rotate very slightly. In order to make it an effective exercise, you'll need to do it repeatedly. Hold for 30 seconds, then release. Do three to five sets of 10 to 20 repetitions. Squat just above your seat. Squats are among the best exercises that you can do for your buttocks, but you don't need to get up and away from your desk to do them.  Instead, plant your feet about shoulder width apart and raise your buttocks an inch or so off of your seat. Make sure to keep your back straight as you perform this exercise. Be careful that the knees stay in line with your shin and don't pass the toes. You should be able to see the top of your shoe at all times during this exercise. Count to ten or hold it for as long as you can. Try to do 10 throughout your day. Work up to a one minute hold or try for three sets of 10 to 15 repetitions. Start out using your hands to help balance by holding onto the arms of your chair or your desk, but work up to hands-free as soon as possible. Tone your thighs with leg lifts. Single leg lifts can help engage your thighs and core and, to a lesser extent, your buttocks. Try straightening one leg at a time and holding it a few inches above the ground. Hold your leg in place for the extent of the exercise, then switch to the other leg. Hold your foot a few inches off the ground with your leg straight. Count to ten before placing it back on the floor. You may want to try holding it up for as long as you can instead of counting. Repeat the process periodically throughout your day. Swap your chair for a stability ball. Using a stability or exercise ball as your chair from time to time can be a great way to engage your buttocks, core and legs while you work. Simply sitting on the ball requires you to regularly activate each muscle group to maintain your balance and posture. You may not want to get rid of your chair. Instead just use the exercise ball as your seat sporadically to ensure you don't put too much pressure on your lower back. Bounce a bit. While sitting on an exercise ball, press your feet into the floor and engage your gluteal muscles to make yourself bounce. Don't push too hard, just continue to engage your legs and rear end by continuously bouncing a few inches up and down. Make sure your feet are about shoulder-width apart to engage your gluteal muscles and ensure you can maintain your balance. Treat your bouncing as though you are doing a series of mini-squats. This exercise will also burn calories during your day. Pivot your body on the ball. Instead of treating your exercise ball like any other desk chair, take advantage of its shape by adjusting your positioning on the ball periodically throughout your day. Keep your feet in place and press out to the left, right, forward and back, using your legs and buttocks to maintain your balance as you move. Hold yourself in positions that you can feel engaging your buttocks. Be careful not to overextend yourself so you fall down or strain your lower back. 
Squeeze your gluteal muscles. Squat just above your seat. Tone your thighs with leg lifts. Swap your chair for a stability ball. Bounce a bit. Pivot your body on the ball. 