One of the thins that Sifu Bruce Lee stressed was that you should "Curl into your sit ups rather then having a straight back, and lower your body VERY slowely, he used to do sets of 20 sit ups, and he would do them soooo slowly that 100 reps was impossible, he also stressed that working a sit up over 30 degrees took the work off the stomach and onto the thighs. .... also when doing the leg lifts keep them between 1 = 9 inches off the cround .. i.e raise to 9, then bak to 1, never put the legs back on the ground. Bruce beleived that you should work from the bottom of your abs to the top. This is his abdominal workout which i follow religiously as a practicioner of JKD.
his daily abdominal workout included:
Waist twists - four sets of 90 repetitions.
Sit-up twists - four sets of 20 repetitions.
Leg raises - four sets of 20 repetitions.
Leaning twists - four sets of 50 repetitions.
Frog kicks - four sets of 50 repetitions.
Suspended leg raise - four sets of 20 repetitions.
hope this is of some help.
Lee further developed this routine, adding additional sets of sit-ups, side bends, leg raises, "flags," twists and back bends to his abdominal workout regimen. The "flag" exercise was a particularly difficult drill Lee devised for working the abdominal. While lying on a bench, he would grasp attached uprights with both hands and raise himself, supported only by his shoulders. Then, with his knees locked straight and his lower back raised off the bench, he would perform leg raises.
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"i said empty your mind. B formless shapless like water. U put water in a cup, it becomes the cup, u put water in a bottle it becomes the bottle. Now water can flow or it can crash ... b water my freind" - Bruce Lee