Before Dr. Martens became a footwear cultural icon, they were the comfort shoe of the working class. Dr. Klaus Maertens invented the sole of the boot in 1960 and developed the boot with a shoemaking family in England. Eventually, postmen and others who spent a lot of time on their feet bought the shoe. After a while, the boot was discovered by teens and is now the preferred footwear of many, both young and old. They can be tough to get used to, however, if purchased new. Some time spent working on the leather can yield a softer boot.
Rub mink oil or neat's-foot oil on the outside of the boots. Make sure to use extra oil on areas that are particularly stiff.
Flex the boot with your hands a few times before putting it on.
Spend a lot of time in your new boots over the next few weeks. Avoid rough terrain but walk around in them a lot. This will help them wear and stretch in the areas you need.
Reapply the oil after wearing the boots. This will help them maintain the shape you are forming in them.
Avoid alcohol-based boot "softeners" and petroleum jelly. These can damage the leather over time.