I stretch my feet up every chance I get. I stand around at work a lot so whenever I have nothing to do, I will put my foot up against a wall (toes pointing up) and lean onto it. Or just put my heel on the ground, toes up... and keep it like that for a bit. Also, doing calf stretches... or sitting on the ground and grabbing your feet and pulling them toward yourself (if you're not flexible enough to grab them.. get some rope or a towel or something and use that to pull them back. )
If you stretch out the muscles you use to get them in that weird position.. that may help.