Meet the BootBra: an elastic band that you can wear around your leg and snap to the inside of your boot to prevent it from sagging. You can use BootBra with different boots. It is 11 inches long and can be stretched to 20 inches. BootBra is pretty easy to use. Just slide it over your foot, attach it to your boot, and you are set.
The above GIF shows how to use a BootBra. Costs $25 at the time of this writing.