With low heeled boots, if you allow your foot to flex and your ankle to bend, the boots will wrinkle, and there's no going back. If you wrapped a stiff piece of cardboard inside while you wore them, it would keep them straight but then your foot/ankle couldn't bend which might not be that functional, or comfortable. Certainly you want to store them with cardboard stiffeners.
You could wear significantly higher heels so there is less tendency to bend the boots at the ankle while walking but eventually there will be some bending.
I fill in around the ankle to minimize the slumping, either by using heavy knee high gym socks wrapped around, or with a couple pairs of dancer's warm up leggings scrunched around the ankle for maximum fill.
The trouble with our tapered calves and gravity is that every movement wants to move the boot top down, and nothing about the system wants to lift it up. At the extreme I suppose you could devise some suspenders. Would you wrap them like garters around your knee? Hang them from your belt? Or run them all the way up over your shoulders?