I think the most important thing is to understand the different types in the category. It just comes from looking at a lot of shoes.
kitten heels, pumps, high heels, stilettos, platform heels, wedges, platform wedges. They all have their unique proportions based on years of shoe making. The proportions can be tweaked and pushed, but they have to be understood. Never leave your sketches alone. I love scanning my sketches, mirroring them and then using the liquify tool to push and pull the proportions. You get different looks quickly that might serve as an underlay for the next sketch.