This has made all my Marlows comfortable right out of the box. I take shoes with really stiff soles and give them a good "pre-flexing" before wearing regularly. Grab the toe in one hand and the heel in the other with the sole up and basically bend the shoe to a 90 degree angle a few times. Don't worry, you won't ruin the shoe even though it feels like you shouldn't bend it that much. For shoes that are too stiff to bend in your hands, stand the shoe vertically with the toe on a soft surface like carpet and press down on the heel/heel cup.
CAUTION! Did this with the Penny and the process creased the shell just outside the heel counter. Only bothered me slightly, but severely dented resale value.