What does Harry Potter learn so Voldemort cant read his thoughts in the order of the Phoenix?

He was suppose to learn occlumency from Snape to keep Voldemort out of his thoughts. But he never did get the hang of it. It turns out though that Voldemort can not poses Harry though he tried that at the ministry of magic. Because Harry poses some force in such a great amount that Voldemort has none of. It makes it so Voldemort can not abide possessing Harry.