You can tighten those stomach muscles without doing those dreaded and painful sit-ups (and hurting your neck in the process).
Step 1: Sit in a chair with your feet flat on the floor about ten inches apart. Make sure you're sitting up straight with your back against the chair.
Step 2: Rest your hands in your lap.
Step 3: Breathe in deeply through your nose while you "crunch" your upper and lower abs as you push your lower back against your chair.
Step 4: Exhale and relax.
Step 5: Repeat ten to twenty times.
See the complete library of Workplace Fitness exercises.