Yeah, situps are a pretty crappy exercise...most peoples hip flexors are too tight in the first place (sedentary lifestyle).

Your abdominals have basically three functions, moving the sternum, moving the pelvis, and stabilization. Exercises like pushups, squats, deadlifts, standing overhead press etc will work them quite a bit because the have to keep your midsection firm to provide a solid enough base. Crunches work the upper part, reverse crunches work the lower. I did enough ab work to last me a lifetime when I was doing crew so I quit doing any kind...just squats and overhead press and things.

To get a sixpack:

Eat a big breakfast. Most of your carbohydrates in this meal.

Small meals throughout the day.

Eat very little before bedtime.

Walk for an hour or an hour and a half every day.

Needless to say you do need to be at a calorie deficit (burning more than you eat).

Exercises like running and jogging tend to burn a lot of muscle, which is not the goal here.