It's all going to be fruitless if you aren't eating with proper nutrition for your endeavors.
But my favorite ab work is one day weighted (machines, knee lifts, decline crunches, etc.) including supersets, drop sets, and so on, followed a couple days later with unweighted/high rep work.
Squatting helps build nice, thick abs too, but that's out for you non-squatters.
This easy hanging ab routine will smoke you easily:
All are 1 set, all are 6-8 reps each. The trick is to use your ABS to do the work and not momentum. These WILL humble you, but they'll also build some killer ab strength and add some size. It's worse if you have big/heavy legs to lift, of course. And by worse, I mean better
It's tough to cheat too, which is awesome. Best of all, once your abs are done...they're done. No searing pain throughout reps, it's just nice and easy to get through it and then you're just smoked. I love it, because I usually hate working abs if it isn't heavy.
1. trunk curls (knees to face, curling lower abs/trunk and NOT just your hip flexors)
2. half-range (from bar to halfway down) alternating leg raises (hold one foot to the bar, lower the other. then alternate slowly)
3. full-range leg raises (hang from the bar fully extended, lift to touch the bar and go back down. repeat)
4. split raises (hang fully extended, touch feet to bar. pause, and spread your legs as far as you can along the bar. Bring them back to touch each other, then repeat.