Your key to success is the lower shell. To get in, pull apart the lower shell, just under the cuff, and your foot will slide right into the stiffest of boots.
Getting out, same thing, get that lower shell open and you'll come out. It's a bit harder because you're trying to pry your foot out as you're spreading the shell...
Once the first boot is off, use that foot to kick off the other boot by wedging it down behind the calf.
Seriously though, knowing to pull apart the lower shell as you're trying to get in is a verrrrrrry useful piece of knowledge, especially when dealing with stiff plug boots.