I think 6-packs are a result of low body fat, and pretty much just that. It might look better if you do 100 situps a day, but not a whole lot. If you have 15% bodyfat and do 100 situps a day, and I have 12% and do ten a day, my six pack will be better. Why so worried about the 6 pack anyway? If you can lose more weight and still be healthy, so you can run faster, I say go for it, and I guess the 6 pack is a nice reward too. I just couldn't motivate myself for that (the six pack alone). I wear a shirt all the time though.
My advice on weight loss (perhaps not the best, but it works):
In wrestling, I'd eat a large bowl of oatmeal for breakfast with some fruit, have a meat sandwich around 11, an apple about 1, practice 3-5, and have a salad for dinner. (And drink lots of water, except on the day of weigh-ins. But you should drink lots of water all the time) I'd lose 3 lbs a week fairly comfortably. (Most I ever lost was 12lbs.) If I needed to lose more I'd just go off one meal, (meaning skip dinner, and make lunch real small) or two meals (no lunch or dinner, just a salad for brekky).