I taught bro Jedi he correct method to get up to 100 situps in your first try.
The most important thing is your warm up (start with stretches for a full 5 mins then a good run will help, and get oxygen to go where it needs to go) - You have tire yourself out slightly, take a break for a minute or two then start with the situps. do 4 sets of 25 situps with a 1 minute break in between. Do it everyday for a month, but for the first week only do this on alternative days. This will help your muscles to recover... also, don't forget to have the necessary amount of water (2 litres a day) stay off the fizzpop, and eat meat only around once or twice a week. This is paramount. The whole purpose is not to just look aesthetically pleasing, but to build real core strength.
Considering bro Jedi is 6ft4inches tall - he managed to heft his weight with this method. He's now up to 200 situps a day, like he mentioned before and now, his stomach is firming up quite nicely and soon insha-Allah, in another 2 months, he'll be ripped into a six pack.... next month, he'll be doing 300 a day, and continue like that for as long as he can. No more than 300 situps though. That's the limit I impose. After that, if he still has energy, I'll give him some other exercises to do. Or increase his superman reps and make him run on natural terrain to build further core strength.
The stomach is a very important part of your body, it's where all your core strength comes from.