Ok, if you really want some abs, you can do sit ups. @ my school, they make us do sit ups all the time and now the ab lines are starting to come in. ok, so here's what I do when I'm not in school:
1. 50 curl ups (just shoulders coming off the ground. It helps to put your hands behind your head and just use your hands to lift your head up. It really works!)
2. 50 half-ways (Lower back coming off the ground. You can pull yourself up by using your hands to pull on your legs to heave upward when you get tired because some ppl are as athletic as others.)
3. 100 full situps (all the way up. chest touches knees.)
4. repeat steps 1 & 2 (you don't really have to do 100 of each in all. you can do as many as you want.)
you can even do more curl ups than anything else because curl ups are easy but effective. Most ppl don't know this so I just wanted to put it out there. enjoy!