I personally believe you have to do a combination of different exercises to tone your stomach not just sit ups and crunches. you need to be doing cardio to burn the fat and different ab exercises in addition to crunches like the reverse crunch, the bicycle crunch, jacknifes, etc..!! I am trying to tone my stomach as well my goal was 20lbs and i have lost half that so now im doing toning i cant tell you exactly what works but im doing both and hoping for results!! With the 30 Day Shred DVD i did lose a whole inch off my stomach in 1 week!! So i am making progress but i can keep you posted as i go!!
It's not so much that situps and crunches don't work at all, but the thing is if you have a layer of fat covering your abs (b/c everyone, even those who are upwards of 300 or 400 lbs have 6 packs underneath the fat) you won't be able to see any definition. In order to tone your stomach so you can see some kind of definition or shrink your waistline somewhat, you really have to focus on cardio which burns more calories, which burns fat. It's unfortunate, but we can't spot reduce so pretty much you just have to lose weight overall and eventually all areas of your body will get a bit smaller. But if you're like me and genetically carry more weight around your mid-section, the stomach will most likely always be somewhat of a problem area. If you feel better about doing ab exercises theres no harm in that, but you should be focusing mostly on cardio. But like gokuangel said, it's best to do a variety of ab exercises to have the best results, and make sure you're practicing correct form because you can essentially do 100 situps in a row but won't see any type of changes if you arent doing it properly.
As the others have said, you need to do a combination of cardio and toning exercises. My waistline is definitely shrinking and my stomach is tighter.
I do regular crunches, twist crunches, reverse crunches, and some other crazy things to tone my abs. BUT I also do cardio! Walk more than 30 minutes a day, jog, ride a bike, get an exercise video--- anything to get your heartrate up.
If you don't have the means to do any of that, jumprope and jumping jacks, jog in place. There is so much you can do!
And as StarryNight said- if you don't do the crunches correctly you won't see results. I believe situps are TOO harsh on your back. So that's why I do crunches. Anyway, I only do 30 crunches of each "type" of crunch (reverse, twist, etc.).
Thanks for the tips guys - I do have a gym membership but haven't made much use of it lately *cough* - so I will do a combination of cardio and toning. I live in a small apartment and like the idea of crunches and situps because I can do them on days when I don't go to the gym and they don't require any equipment. I also like to do leg lifts because I don't think crunches really work the lower ab 'pooch'. I am 41 years old, 5'3" 145lbs and want to get down to 120-125 in 4 months - 115 would be even better but I don't know if that is possible. I am not following a set diet but eat 1200-1400 cals/day, can't do low carb because I am vegetarian. I will be seeing my husband again after 10 months apart so that is my motivation ;)
The reverse crunch is supposed to help with the belly pooch! Speaking of which i need to start doing more of them than just whats on my shred DVD!! lol!