Training the mid-section is not that hard-you just have to be smart about it. What most people do wrong is that they focus on the same exercises and then wonder why the results are not showing. When training your stomach muscles, there are two main concepts that you have to keep in mind:
1) work them out intensly; high reps few breaks
2) use different movements to target all of the muscles in the region: the transversus abdominis, rectus abdominis, and obliques.
I think that the best ab exercises are those which combine all three movements into one, which forces the three muscles to work together. This leads to a better workout and better results. A great exercise which does this is the V-Sit Up. Here’s what you do:

Start out lying down on the ground with your legs straight out and hold them about 1 inch off the ground. Tighten up your stomach.

In one motion sit up while bringing your knees up to your chest. As you do this, make sure you are keeping your stomach tight and are initiating the movement from the midsection and not the upperbody (you are trying to isolate your abs, you don’t want to cheat by using your upperbody for help).

Once you get the movement down, you want to do it at an angle so as to incorporate the obliques. Same thing as before, except that before you start the movement you want to be slightly angled to one side.

Doing 12-15 repetitions will target your abdominals in such a way that traditional crunches won’t. Try it out and feel the effect that it has on your abs!

Just found your blog linked from allfacebook. This exercise looks great, should make a change from sit-ups. I’m looking forward to trying it out later.