Alright, listen up....
At 140 lbs, depending on your height, your body fat % should be low enough to start seeing your "abs".
Your abs are based on only a few large sheets of muscle - the "lines" that you see with well-defined abs are actually cords of tissue that hold the entire package together. It is when your abdominal muscles stick out past these "cords" that you start to see the "cubes".
Keep on with the sit-ups and focus on technique. Slowly up; slowly down; controlled movement is key.
I don't really believe in sit-ups however, so I tend to suggest crunches. A crunch is a very similar movement, however the focus is not on simply launching your upper body upward, but actually squeezing or "crunching" your abs together to get that major burn.
Another thing, doing 200 sit-ups each day is simply going to build density in your abs and make the muscle harder. While you do want this, you also want your ab muscles to grow a little bit so that they do stick past the cords of tissue and create that beautiful 6-pack look for you. In order to do this, you have to reduce the number of reps and pump up the intensity of the workout. You want to boost the intensity so much that you can ONLY do 20 at a time, after that point your abs should have built up so much lactic acid that you feel that slight "burning" sensation that bodybuilders love so much.
In order to raise intensity to this level, do as described above and either execute your crunches on an incline, or place weights on your chest. DO NOT add weight or increase the incline at the expense of good technique, however!! If you have 60 lbs on your chest and are working on a mad incline, but your are "swinging" your body up, you aren't working out effectively. SLOWLY up, pause, SLOWLY down - C O N T R O L