My coach gave us a pretty good ab workout the other day that hurt like hell.
R1-30 secs full sit ups, 5 sec break at end then....
R2-1 sit up, then sit up 2 twists. 1 sit up, then another with 4 side twists - keep going up to 12.
R3-same as last time but after each set of twists you hold half way up the sit up for 5,6,7 etc secs until you reach the 12 twists set. 10 sec break
R4- same as last time but after the holding bit, you go back down and then up as quickly as possible and then lower yourself back down over 5,6,7etc secs until finished the 12 twist set.
adjust numbers to suite yourself, but the other guys he trains says it works really well to strengthen the abs and obliques etc when getting hit. hope the routine is clear not easy to explain in writing.