rbdude79 saidI like any combination of the following... usually do 3-4 of these exercises every other day at the beginning or end of my regular workout. Sometimes I do two days in a row, but try to rest like any other muscle group.
1) Planks -- mix it up... regular planks, one leg raised, one arm raised, opposite leg and arm raised, side planks, etc.
2) Classic crunch -- can vary this a million ways as well; I usually just do the standard one, sometimes with added weight, sometimes without.
3) Crunch machine (the sitting one, not the one where you lay on your back... that one's useless) -- I like this for building definition 'cause I feel like I get a broader range of motion with more resistance from the plates.
4) Leg raises -- from king's chair, flat on ground, or decline bench.
5) Russian twists w/ 10 or 25lb plate -- great for the obliques.
6) Side crunches w/ 25 or 35lbs plate on 45-degree bench -- also great for the obliques and that V around the hips/groin.
Sometimes I get bored and throw in some other stuff... had a trainer once who had me get in a push-up position with my feet on a knee-high stool... then walk my hands all the way around and do push ups at every 90-degree stop. It's kinda killer.
Snowboarding helps a ton in the winter... lots of core there. Really any full-body outdoor activity is probably a good extra.
Awesome combo there..
Abs recover very quickly and can be done daily if desired
7) Toe taps
Hip thrusts or reverse crunches
9) Reverse bridge with one leg extended
10) T Planks (Side plank with straight arms)
11) Side plank with a kettlebell or dumbell on the up arm.
12) Backward crunch - (From Sitting up to 45degrees) Weighted with a plate.
13) Pass a ball between feet and hands - back to ground.
14) Dead Ant (I can show you what this is if you can't find it)
All tha best.