Here's a few things to try:
Some recommend soaking them in a wintergreen solution (available at your drug store). It softens the rubber a bit, and makes it more pliable.
If it's a sticking problem, try some dishwashing soap and water on the inside of the boots. Once it dries, it does get stickier.
Some say you can use a blow dryer for hair to warm up the ends, making them more flexible.
Do be careful with this recommendation. Open spark from the blow dryer, mixed with stray gas fumes, can be a problem.
I hope this helps. Let us know how it turns out.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.