Find a stair and place the balls of your foot at the edge to where your heel hangs off the side, then sink far into your heels like you would while riding. I did this motion up and down holding for a few seconds at a time for about 10 minutes each day. It's also a great calf workout!
Condition them so they become more supple(especially around the ankle). wear them around the house before riding, wrap your ankles in vet wrap to avoid blisters once you start riding, use a moleskin if you do get a blister. As a last resort you can get them professionally stretched my a cobber.