while standing with dumbells in both hands at your sides, slowly raise them until they are at eye level, then slowly back down. this works back muscles and arm muscles more than your abs though.

if you're at the gym, there is an excercise machine where you push back a lot of weight using your ab muscles. try that machine if they have it. i could do all the weight on the rack when i was a freshman (which was about 500 lbs)