Corple: Abs/Trunk/Core muscles are different then the other muscles of the body. Even if you're trying to bulk up your chest/back/shoulders etc. You need to train to have a strong core not a big one. Focus on activities that will benefit you during traditional weight training. Do swiss ball sit ups, medicine ball twists, side bends, knee raises, bicycles etc. Vary the reps anywhere from 6 - 25; do 3 or 4 exercises, 3 sets each twice a week and you'll be all set.
Also: To challenge your core even more you can incorporate core training into your weight program. Any activity you do seated; do standing to challenge yourself more. Anything done lying down you can do on the swiss ball. The only warning is that you won't be able to do as much weight on the swiss ball, but you could still incorporate it. Say for chest: Flat bench, and then do some dumbell chest presses on the ball. This will really challenge your sense of balance and core.
One other fun thing: When doing standing or ball exercises; use a lighter weight in one hand (IE 35 and 40) this will really challenge your sense of muscle balance.