Boots can be a fashion statement or be worn for practical reasons, such as when you are walking in thorny, muddy areas or anywhere snakes visit. Many boots become more comfortable the longer you wear them--they mold to your feet after awhile. However, you can speed up this process, and make your boots more comfortable.
Use boot stretchers. You can find these devices at stores that sell shoe supplies--particularly leather shoe supplies. To use these, place them inside the boot and adjust the stretcher to apply a little tension on the inside of the boot. These work best on leather boots, to keep leather from shrinking when you aren’t wearing the boot, but you can try them with synthetic materials, too.
Insert gel insoles. Many boots have little in the way of comfortable insoles--many are not even cushioned. Gel insoles take much pressure off the bottom of your feet, providing a cushioning effect. These may also help with the fit of the boot, if the boots are slightly too large (the insoles take up some space).
Condition leather boots with leather conditioning cream. This prevents leather from becoming too stiff and may make the boots feel more comfortable.
Wear, and walk in, the boots for at least an hour a day after you buy them. This is a gentler breaking-in process than wearing them all the time initially. Boots, synthetic or leather, will gradually adapt to your gait and the size and shape of your feet, provided the boot is not overly large or small for your feet. An hour a day is enough to gradually break boots in without causing a lot of agony to your feet in the process.
Wear good, thick socks with your boots. Thin socks, and especially if they are made of synthetic materials, allow the foot to slide around in the boot as you walk, which may cause blisters. Even if your boots are dressy boots, good socks are important.
Hiking boots (and other boots), according to rei.com, will naturally be stiff at first.
One way to get boots to mold to your feet quickly is to soak them in water and wear them until they dry--but know that this will likely ruin many kinds of leather boots, leaving them prone to cracking.