That six pack is divided into 3 sections: upper, mid and lower abdominals. Do some incline crunches to hit the upper, flat to hit the mid and some leg rasies to hit the lower. work a little oblique in there as well. Muscle and Fitness has a Stomach edditon of the mag that you can order online. If you are serious, get it. It has over 50 stomach exercises of which 20 are really worthwhile.
The other thing is to consider your diet and cardio. Without both of these in check you will have a six pack, you just wont see it through the bag of loose fat skin.