Ab day is always strange for me. I want to feel like I get a really good workout, but it is so much different with my abs than any other muscle group.
For most of my workouts I continue my reps until failure, until I literally cannot do another one. My core is not very strong so I fell like I get to this point very quickly. But I also have a quick recovery time with my abs and so just a little while later I feel like I could do so much more. So here is my solution.
Four Superset Circuits, Repeat each individually until failure (except #2 and #4 - repeat twice before moving on), do cardio for the day, then come back and do it again.
I actually feel sore in my abs today! I workout my abs twice a week. I work until I can't do anymore but I have never felt sore before. This is definitely a keeper!!