Shoulders back, Raise your hands up his neck more and overall just go forward more. Sink ALL your weight into your HEELS. You really need to give more of a release. Try looking at pictures and videos of people jumping to help you get an idea of what you need. Here are some to look at: Jumping position - Wikipedia, the free encyclopedia
This is a really good article.
But really, watching videos on youtube of people jumping really does help!