A pair of binder clips is all you need to keep tall boots from bending at the ankle and folding in half. Whether you want to save space storing your boots, or you just don't like the idea of a permanent crease at the base because the top flopped over, this trick will net you both benefits.
Just use a binder clip or two at the top of your boots to keep the pair together and upright. This keeps them from flopping over and limits them to the floor space required for their actual footprint. If you use really strong binder clips, you can even hang your boots this way (although that will put some stress on where the boots are connected by the clip). It's a simple, space-saving solution, especially if you still want to use them and aren't ready to box them up and put them in the closet for the season.
If you're no fan of the binder clip method, WonderHowTo has a few other boot-saving, space-saving tips you can try. Hit the link below to check them out.