I have clear plastic stacking pigeon hole type things from the Holding Company. Each one holds either one pair of high heels, or two pairs of flatties/kitten heels. I have them stacked on the shelf in my wardrobe, and another stack under the bed. They sell them individually but also in a pack of six with a plastic lid so you can have them out of the wardrobe and keep them relatively dust free.

Mazl, I have some boot trees which keep my knee boots upright - you can buy them at John Lewis or many heel bars and shoe shops also sell them.