There is a small book called the 'Panandata guide to knife throwing' that discusses how to adjust the release point in relation to the balance point of the blade you are throwing. Its a nice text, and helps give you a good chance of success.
As Brian said, a knife designed for throwing is the best tool for the job- they have better balance, and tend to be of solid construction- no handles to go 'wobbly' when you inevitably flat-slap the target for the 80th time

Relax, and throw from the whole body, not just the arm, same as with a strike, keep to the same knife, at the same range and with the same throwing motion. Dont keep changing the conditions or you will never stand a chance.
If its any consolation, its very tricky to do, and I am rubbish at it, even with the books and the theory in place.
