RE: short but effective abs
I do 4 sets of 5 pause-with-static-hold reps once a week, weighted. I sit on the edge of a raised floor, feet held, weight on chest. I lay back so I'm horizontal (but body still unsupported as I'm sitting on the edge of a raised floor), hold that position for a count of 5 and repeat for 5 reps.
Doesn't take long at all.