Remember to use your arm too. A basic jade split is like basic scorpio and gemini – you are using your underarm and side body to hold yourself in place, not just the inner thigh. Gripping with just the inner thigh is a more advanced version of jade and is more difficult.
Basic:
http://images.quickblogcast.com/6/4/6/2/0/310038-302646/Cleo.jpg
Advanced:
http://www.poledance-paris.com/12_repertoire/images/planche03_fig26.jpg
Veena has a floorwork lesson called the inverted floor split. It is basically a jade split done from a shoulder stand. I found that practicing this move really helped me get the hang of the balance in the split position. It made doing it on the pole easier. My jade isn't flat, but it's getting there.
Good luck!