Core Crusher Finisher Circuit
Here is core circuit using plank variations that i used as a finisher with my conditioning for fighters class.
https://www.youtube.com/watch?v=l177Tl2p2Ps
Core Crusher Circuit
Perform each exercise for 30 seconds followed by a 5 second rest.
Complete 1-2 rounds
(Take 60s rest if 2 Rds)
Exercise List
1. Plank
2. Plank Alt Arm Raises
3. Plank Body Raise
4. Side Plank Leg Lifts (Right)
5. Side Plank Leg Lifts (Left)
6. Plank with Alt Leg Lifts
7. Spiderman Planks
8. Side Plank (Right)
9. Side Plank (Left)
10. Walking Planks
Funk Roberts
I follow 7 minute ahbs. The great thing about it is that if you don't get the burn in those 7 minutes they will send you the extra minute free. 
I just recently got back into training and lifting after a serious accident in '12, and it was always easy for me to ignore ab exercises because the average crunch or sit-up was always so easy to perform for me. My name is still on the weightroom wall at my former high school for holding the record for decline situps with most weight on my chest, ten years later. Pretty cool.
Lot of helpful info here, some stuff I'm definitely gonna try out. My equipment is very limited, so it's mostly bodyweight exercises. I didn't work out my abs for two years in July, now that I'm getting back into shape, it's 100 sit-ups a night for one week, then add 50 a week. I was planning on switching it up with some leg lifts with ankle weights, and twisting sit-ups. Now I have plenty of other options as well. Thanks guys.
I do direct AB work about once every three weeks...
TAKU
Dragon flags, hanging leg lifts, windshield wipers