Don't do high reps on abs. That is pointless. All it will do is burn and (maybe) increase endurance. Keep them in the 6-8 rep range for mass and strength. Abs already have plenty of endurance (they support your trunk all frickin day after all) so what you need to focus on is STRENGTH.
Make sure you are already doing deads, they are good for core work. Then try weighted crunches. Those get easy real fast, so you can try holding your arms over your head, holding a plate to make it harder if you want, or use a weighted vest or something. Cables work too, I guess.
Incline leg-hip raises are great too. Lay on an incline and pull your legs up to your chest, bending your knees, so that your butt just comes off the mat. You should feel it directly in your abs. This one hits your abs directly, not your hip flexors, so it's a good one. Adding weight on this one is a bit more difficult though.
You can hit abs frequently, but depending on what kind of workout scheme you are using, just do them whenever you do deads or whatever.