Naughty by nature
Well iv always had a really good eye for that stuff. But its also a lot of practise. Im not as good as i used to be so i guess i need to go back to life drawing classes. But for a little advice try lining up parts of the body, well first up try the face for example or a simple object like a bottle, say line up the edge of the bottle cap with the bottom corner of the bottle then do the same for other parts and you will then form an idea of proportion. just mess about with it and use ur pencil to line stuff up!