Mike, to improve the definition in your abs I have a very good excercise, it's really hard to explain though, here goes...
lay down on the floor, arms and legs extended, so your arms are on the ground over your head. Then arch your back, you will feel tension increasing on your abs. This means your abs are extending further than normal, your ROM is increased. Now raise both your feet and hands a couple of inches off the ground, still having your back arched. You now contract your abs, forcing the arch out of your back, you are effectively pushing your back to the ground this way. it also means your arms and legs will go up in the air. Do this very slowly, on both the positive and the negative part of the excercise. This excercise kind of grafts the skin on your abs. Do it every other day, don't count reps, make sure one rep takes around 10 secs, 5 up and 5 down. It's tough at first, but it such a great shaping excercise and it strengthens your core a lot!