Sit upright in your chair, keeping your backbone erect, and look straight ahead. Contract your abdomen as hard as you can and hold it for 4-5 seconds. Release and repeat in 2-3 sets at a time. An isometric exercise that does not require continual movement; it exercises the abdominal walls and vibrates the organs of the stomach cavity, keeping them active. Also, it can be done while you work on your computer.
Grab the edges of your chair or work desk and walk your feet in towards the back till your body forms an angle with the chair or desk (depending upon which of these two you're using for support). Keep your toes hip width apart and lift your hips so as to align your shoulders and heels in a straight line. Hold this position for 35-40 seconds and return. Repeat in a couple of sets at a time. These lifts are very good for working and strengthening your abdominal muscles.
Sit near the front edge of your chair and stretch your legs out in front of you. Grab the edges of your chair behind the seat, lift your legs about 6-8 inches off the floor and lean back slightly. Now lean forward, pulling your knees into your chest as you do so. Squeeze tightly and stretch back out. Repeat 3-5 times. Working the lower abs, they are among the most motion-oriented lower abdominal exercises at your desk.
Belly Rise and Dip
Sit up straight, keeping your backbone erect, and imagine your belly button to be an elevator at the top of a 6 storied building. Lower the elevator to the 1st floor as you breathe out and hold it there till a count of 10, breathing lightly all the while. Repeat thrice a day. A Pilates exercise, it incorporates a sync between respiration and abdominal muscle activity and prevents your belly from "brimming over".
Sit tall with your knees bent and the soles of your feet flat against the floor. Placing your hands at the sides, behind your head, curl your torso downwards, squeezing your abs tightly. Return and repeat. Traditional abdominal crunches can be modified to suit your workplace limitations to give you the same benefits.
Sit with your backbone stretched and upright; hold the edge of your desk with your fingers and thumbs; feet flat and firm against the ground (adjust your chair's height so that your feet do not dangle loosely). Suck your stomach in and use your core as the axis to swivel the chair from left to right and vice versa. Do at least 4-5 sets of 20 rotations (1 rotation is 1 left-right-left set). Imagine a rotating tub of washing machine and you'll get a better idea.