hey there sebass,
I used to have the same problem, felt like I had to throw my upper body forward to get off the box.
That problem got gradually better by using a very wide stance (feet currently touching the insides of a cybex power rack) and wearing a z-suit with the straps down. If you don't have a suit, get one, they protect your hips, and let you squat wider and sit back more. They are also pretty cheap, like around forty bucks.
The problem wasn't solved overnight, but right now I don't feel like I have to lean forward at all to get off the box. Off course that may be just because I have been doing it for so long I don't notice that I am doing it anymore.
B