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!
Artists have known for centuries that the average human adult body is made from parts that are in proportion to each other in predictable ways. If you draw your figures according to these ratios, your finished work will look more natural.