Hey,
Inversion can be tricky and should be tackled in different steps, it usually doesn't hapen all at once
.
Try swinging up perpendicular to the ground on a bar or rings, make sure you swing from the shoulders and not hips this should help you get the feel for swinging from a hang and not teach you to pull yourself up.
http://www.youtube.com/watch?v=Gn5LIiHmPlY (video is not mine just grabbed from youtube)
After you get the feel for the swing try running from a short approach and swing your trail leg through to your top hand and land on your back in the middle of the pit. The invert can't be fully reached until you know how to whip your trail leg through after take off. Getting your hips up is the key to inverting and those two drills can really train you how it feels and how to do it.
Here's a video of this drill:
http://www.youtube.com/watch?v=hsDq3ZZd6tQ (video is not mine just grabbed from youtube)
Hope this helped