My nemesis. Sucks to work them. Feels so good when you're done.
Found this routine that I modified slightly that seems to work well and it's not too long...about 5min.
20 of Each of the exercises:
-Crunches (touch right elbow to left knee)
-Crunches (touch left elbow to right knee)
-Bicycle (alternating back and forth between the previous 2 moves)
-Upper Abs Crunches (lay on your with your legs fully extended, arms fully extended above your head - trying to be as LONG as possible. Then just come up to about a 45 degree angle, enough to tighten the upper abs)
-Lower Ab Crunches (lay with your feet on the ground and legs at a 45 degree angle. reach your flat palms to the middle of your thigh, then roll up so your hands touch your knees, just flexing the lower part of your abs)
-Reverse Crunches (Extend legs and "curl" them up so your knees crunch towards your chest)
-Crunches (legs extended and straight up in the air, hands behind head and elbows out)
-Plank (keep back straight, tighten glutes up and hold for :60 - twice)
That works a pretty good section of your abs...have fun and let me know how it goes!