I don't think you can. The colder it gets, the stiffer your boots will feel, and I dont' know if there's anything you can do about it. I think when buying boots, you really need to take into account what kind of skier you are (level, racer, freerider, jibber, learner, etc.). Then translate that ability and style to what boot will best accomidate not only your skill set, but what type of skiing you do.
Do you race?? Look into a nice stiff boot that will support you at speed. Are you a big mountain powder ripper? Look into a freeride boot that will support you at speed. Are you learning? Look into a boot that you'll be able to control the flex of, because chances are you don't need the support that a racer needs.
Best of luck.