Hanging Leg Raises for abs - can't beat 'em!
Don't swing or use any body momentum to bring your legs up. You can start off building up strength on this exercise by doing them first with your knees bent and then progress to doing them straight leg:
http://www.shapefit.com/abs-exercises-hanging-leg-raises.html (Lotsa great ab exercises here.)
http://www.youtube.com/watch?v=G0ysNevIv0w (Very good form, you can also use a supinated grip - palms facing each other - much less stress on your rotor cuff while hanging - if your gym has these grips.)
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