I work them when I do cardio...after my cardio. So for me, that would be 2-3 times a week.
Of course I know you've heard this one about 1,203,309 times...
"abs are made in the kitchen"

SO TRUE.
Anyway...speaking personally...I find I get more of a burn if I concentrate on my ab muscles when I'm crunching or whatever. Here are a few of my personal ab tips...
1) While crunching, try and push your belly button to the floor...and keep it there...know what I mean?
2) I hold a five-pound weight behind my head when doing floor crunches - when I'm using the Roman chair I hold the weight (or a medicine ball) at my waist.
3) I don't rest between sets.