In addition to laying a sheet of 4 mil poly over the floor of my garage, I cut a piece of rigid insulation for each tire and drove the car up on them. Not sure if this was necessary to prevent flat spotting or not, but it can't hurt. I added fuel stabilizer to the gas tank and topped it up with 93 octane fuel. I also hooked up my car to a battery tender to keep it fully charged.

 

Here is Evil's excellent thread on winter storage. It is very helpful:

https://mustangforums.com/forum/classic-mustangs-tech/500111-evil-s-winter-storage-tips.html