Exercise Myth #4 - Doing sit ups is the key to getting flat abs
When you do a targeted ab exercise like a full sit up your back is not supported. In addition to the potential back strain, your core muscles were not designed to take a significant amount of crunching, twisting, and bending. The real role of your core muscles is to stabilize and support your mid-section and protect the low back.
The real key to a flat belly is to first burn the fat (interval training is a good choice) and then try exercises like mat pilates or balance work. Notice picture #1 - she's pulling on her neck and her low back is not supported. Picture #2 is a pilates exercise - you are lengthening the core muscles while strengthening them as well as keeping the low back down and supported. As long as you push your abdominals muscle tight into the floor and your low back doesn't arch, you will see the most significant benefit from these exercises.
Also, any exercise where you're working on balance will recruit all the muscles of the core (upper abs, lower abs, obliques, and lower back) Some examples: Exercise Ball, Bosu Ball, TRX, or simply standing on one leg and using your core muscles to stablize you.
Give it a try, you won't be disappointed!