You want to keep you shoulders, hips and ankles in a straight vertical line, so work on keeping your legs bent more so your ankle sits underneath your hip.
I'd just like to comment on the hunched shoulders: to fix this, imagine you have a star on your chest, stretching from shoulder to shoulder and to the end of the sternum (chest bone). You want to show this star off, so keep your chin up and shoulders rolled back.
Another thing that I would like to suggest is that you imagine a string going from the top of your head to the ceiling, pulling you upwards. Sit up tall and straight, and, again, shoulders rolled back.
I always found that the star trick worked for me - my trainer would shout "Show off your star!!" whenever I got the slightest bit hunched.
Hope this helps