Unfortunately you can't spot-reduce, which means reducing the fat in one specific area and making it leaner. You can build the muscle in one targeted area, which may be what you're looking for. But the look that most people want when they say "toned"-- i.e. flatter and leaner looking-- comes from diet, not targeted exercise towards that body part.
A lot of fitness professionals are moving away from crunches and situps because they aren't ergonomic and not good for your back. Think about it- we sit in hunched positions in front of our computers or on the couch all day and it's the reason for so many back problems and poor posture. Why do an exercise that essentially replicates that position?
If you are looking to strengthen your abs, try planks instead of sit-ups. It requires engaging the core in conjunction with the back, arm, and leg muscles. Here is a picture:
http://www.urbanfreeflow.com/useful-exercises-the-plank
If you really like crunches/situps, perhaps you can try them with a Swiss ball, which gives you a much wider range of motion in your situp.
There are tons of other exercises out there that are more effective, and better for your spinal health. Another is mountain climbers:
http://www.livestrong.com/slideshow/...ium=1#slide-17. I'm sure others here have more ideas.
Good luck!