Leather boots -- and shoes in general -- are often a little stiff out of the box. With boots, however, it's a little more complicated, since the shaft that extends beyond your ankle and over your calf might be too stiff to wear them comfortably. Fortunately all leather stretches, and you can expedite the process to keep that new pair of boots consistently in rotation.
Condition your boots using one of the many available leather conditioners on the market. These products penetrate the leather fiber, keeping the material supple and more flexible. Apply the conditioner using a slightly damp cloth, rubbing it into the outside layer of the boot.
Apply mink oil to your boots. This compound mimics the natural fats found underneath the flesh while the animal was alive, which progressively become lost as leather ages and is treated with various dyes. Although mink oil often slightly darkens lighter leather, it also adds a degree of water repellency and a nice muted sheen. Rub the oil in with a clean cloth -- preferably dedicated to the task -- to avoid mixing it with polishes and other cleaners.
Lightly spray the boot shafts with distilled water. Put on your boots, and wear them normally. The moisture helps with stretching and molding the shaft to your leg. Don't over-saturate the shafts, as this can make the material brittle over time and exceedingly uncomfortable to wear.