Variety of exercise is the key I have found. For example..an exercise called Mountain Climbers.
Mountain climbers are done by starting in a pushup position
and then shuffling your feet in and out so that your knees are
moving in under your chest and then back out to starting
position. It sort of resembles climbing a mountain but flat on
the floor. If you want an advanced version, you can also shuffle your hands 8-10 inches forward and backward in
addition to the leg movements. This really makes it a full
body exercise and MUCH more difficult than standard mountain climbers.
After finishing each exercise, rest about 30 seconds before
starting the next exercise. Rest about 1-2 minutes after
completing each "tri-set" before repeating.
This will give you one of the best ab workouts you've ever had
without even doing any direct ab exercises. You'll see what I
mean after you try it!
Once you get away from the myth that a 1000 crunches and 500 situps a day is all you need to do for great abs, then amazing results can be seen.
There is a completely free 65 page ebook about exercise, nutrition and abs and stomaches in particular at
http://www.projext.co.uk that you can download (no catch!) and it is excellent.
It helped me greatly.