If you can sit by the TV and do 100 or more sit-ups, you’re most likely doing them incorrectly. The right way is as follows:

Exhale as you lift torso. Keep a fist’s distance between chin and chest. Focus your mind on the muscles, keep constant tension on abs throughout the movement to make sure you’re not using legs to pull you up. Keep a neutral spine:

Lie flat on your back and tilt your pelvis forward, increasing the curve in your back, now tilt your pelvis backward, rounding your back against the floor. Let your pelvis rest in the middle balanced between these two extremes; this is neutral spine. Keep neutral spine for all these exercises.

1. The Plank

Rest on elbows and knees. Have elbows just below shoulders. Keep tummy tight. Hold for 30 seconds. Repeat twice. Feeling adventurous? Go to level 2!
Start on toes and elbows. Hold for 30 seconds and repeat twice.

2. The Crunch

Lying on back, hands resting on shoulders. Using stomach muscles, lift shoulders off floor. Hold for a second and lower. Do 20 repetitions. Want to take it up a notch? For level 2, do the same as above, but have legs in a table top position.

3. Cycling

Lie on back and have knees bent, legs above hips at a 90 degree angle. Lift and twist opposite shoulder to knee. Hold for a second at the top and repeat other side.
20 repetitions.