I find that a weight machine works best for abs... the one with the bar above your head and u pull down. Put some weight on that and do some curls... I got a 6 pack in like 6 weeks. What I do is I get home from school, lift weights then eat. I go hang out with my friends come home lift weights and eat... every day. I feel guilty if I dont work out before I eat. I eat a LOT of wheat and bread and meat.
One of the best exercises I have found for those people who cant get to to gym every day, just do a normal crunch but lift your legs off the ground and bend your knees so they are parallel with your body do about 5 sets of 20 you'll be pretty sore when you wake up. I wouldn't change your diet unless your eating chips and candy bars every day.
i think you should be less concerned about how nice your stomach looks and concentrate on how it performs more. you want strong abs, whether they are visible or not doesnt really matter, that will come with time.
i used to do circuit training for my abs, which is good for endurance, but now i do weight training. take a regular weight (the kind you put on dumbells and barbells and stuff) and lay on your back and lay it on your chest, cross your arms around it, and do 3 crunches, take a rest. do that 6 times. then switch to doing oblique crunches, then sit ups.
use the most weight you possibly can but still be able to get through the exercises. thats the only way to excell, push yourself.