I learnt by applying principles of observation I've learnt from books and few art lessons (and I have to stress how little lessons I've had; I've had only four lessons from an Atelier Studio, but Life drawing is really great as a learning asset, that certainly beat a mediocre course at SQA Standard grade.)
The books that really helped me; were Betty Edwards
"Drawing on the Right side of the brain" which explained creativity and how to foster a mindset that'll help your spatial awarness into actually observing and understanding real forms, and a final chapter that explains briefly how letting fantasy have a basis in real life, makes it stronger and more presentable. Then there's
"Dynamic Anatomy" by Hogarth, I'm summarised key points on general areas on notes I'll use for reference as I'm kind of prone to drawing deranged or broken limbs of unequal length. (There's also Loomis, who you can get every one of his books from this handy source:
http://alexhays.com/loomis/ as most of his books are out of print, I had the same problem with Hogarth for a while.)
I'd advise not reading anything by Christopher Hart, and
certain "How to draw Manga" books which were very sort of 'empty' in their instructions or prinicples other than copy this cute, sexy uniform or this idealised body type, although there was one spin-off series I've read which taught "proportion" which was useful explaining variances you can do if working the body by the length of head. Like the Canon Man is X heads tall. (It was also good to learn to draw chibi characters, which work great as mascots. ^_^)