How to Draw - Study: A Guide to Heels for Comic / Manga Panel Design Reference. Although I'm not a fan of high heels myself, this is an interesting reference for drawing the correct heel position when designing female characters.
Anatomy basic shapes - most people know of these simple drawing rules. But when you are having fun drawing you somehow ignore them, or maybe thats just me? Anyways, i beleive using them more consistently would improve my drawings a lot!