Here's a quick little exercise you can do in your office chair that will help tone your waist and abdominal muscles. If your chair has wheels, place the chair back against your desk or the wall so that the chair won't roll.
Step 1: Sit up straight with your feet flat on the floor and your butt at the edge of your chair.
Step 2: Place your hands on the sides of your chair, next to your hips.
Step 3: Lean back in your chair, but not all the way. Your back should not touch the chair.
Step 4: Raise your feet off the ground, keeping your knees bent.