According to author Shirley Archer in Fitness 9 to 5",Spending a few extra minutes each day in your invisible chair tones up your abs, back, and thighs. Building more muscle mass boosts your metabolism for more calorie burn even while you sleep!" Consider doing this exercise while you're talking on the phone or reading the mail.
Step 1: Get up from your desk and find a wall to press against.
Step 2: If you're not holding on to anything else, consider holding a weight in each hand.
Step 3: Slide your back down the wall until your thighs are parallel to the floor, just as if you're sitting on an imaginary chair. It's okay to hold on to a desk (or co-worker) to take off some of the resistance.
Step 4: Make sure your knees are directly over your ankles and your head is against the wall.
Step 5: Hold for at least five seconds. Over time, work up to one minute.
If you're feeling really creative, try lifting one foot off the floor for awhile, then switch and lift the other foot. If you've got knee problems, try going just half the way down to the full sitting position.
See the complete library of Workplace Fitness exercises.