It depends what's causing the fat.
If it's general bodyfat from being overweight, then the best method is probably to reduce your carbohydrate intake and increase your exercise. You'll need to adjust your insulin intake accordingly. This approach will reduce the amount of fat your body stores and increase its metabolism of your existing bodyfat.
However, sit-ups and crunches will do nothing for reducing your body fat. Fat burning isn't localised ie. if you exercise one part of your body, the doesn't disappear from just that area. If this were true, we would all have rigid lantern jaws and gaunt faces from the amount of chewing and talking we all do! Crunches and sit-ups will tighten the muscles in your stomach which can help with stomach toning but if you have a sizeable layer of fat on top of these muscles, you won't see the benefit.
The other possibility is if the fat on your belly is a result of repeated insulin injections. If you repeatedly inject in the same area and/or don't change your needle, you scar the tissue in your stomach which creates fat deposits that are VERY hard to shift. The only real way to beat this is to stop injecting in that area entirely and you'll probably need to leave it for as long as you've been injecting there. For instance, if you've been constantly injecting in your tummy for the last 10 years, I think you'd probably need to leave it injection free for another 10 before it gets back to normal.