I train abs a little bit everyday after I finish my boxing training. Here are my three favourite exercises I use to really get at those abs. I should warn you they ARE hard, but then again nothing worth it comes easy!

Declined weighted sit-up. I find it more effective than the common sit-up because of the fact that it's on a decline and so you have the added pressure of gravity and with the weight it makes for a challenging exercise.

Side bends (with weight). It targets the obliques (side abs). Note here though that you should really try to bend more than this girl. When you bend, reach down low, as if you're trying to touch the floor.

And this one is a killer but it REALLY targets the lower abs; it's the hanging leg raise.