What works well for me, is hanging knee-raises.... except I do them a bit different than most.
Hang from the chin-up bar and bring your thighs up parallel to the ground.... (ie. your thighs are at 90 degrees to your body).... that is your STARTING POSITION. Then without swinging, bring your knees up as high as possible towards your chest.... and slowly back down to parallel.... don't lower your thighs past parallel with the floor, or you're using hip flexor muscles like ilio-psoas, iliacus etc....
That's what works for me. Certainly not an EASY exercise, but you'll find that the most effective exercises aren't the easiest ones.