try doin your abs inbetween sets. you should be hittin 12 sets of 25 or so for your abs and doin them any day they aren't sore if you want them to look serious.
say you are doin bench....after you rep out turn around and hook your feet down on the floor underneath and hang your shoulders off the front of the bench and do crunches.
or when you're doin tricep pushdowns use the rope and drop the weight and kneel on the floor and do some pulldown crunches.
or hit the roman chair.
or hangin pikes
i try to do four different types of ab exercises every third day or every other day if i can. and mix it up as much as you can.