New handgun holsters are usually stiff and tight. They should be, because the leather will mold itself to you and your use like a baseball glove or shoe. You may be tempted to speed up the break-in with a heavy dose of neatsfoot oil. But that will leave the leather soft, making for a difficult draw. A quick survey of holster manufacturers, and a trip to a gun range, revealed some more effective methods.
Empty your handgun and the clip of semiautomatics of any bullets.
Push the handgun deeply into the holster, until you can push it no more.
Wear the holster and handgun around the house, as much as possible. This will speed up the holster conforming to the gun and to your body. Wear a belt holster with the belt you usually wear.
Draw and holster the pistol whenever you find a few moments. Only through repetitive use will drawing and holstering become easier. Work any catches or snaps as well, to break them in also.
Twist the holster on occasion, perhaps once or twice a day for the first week, flexing the leather one way and the other. This will loosen the collagen fibers (the tough inner structure of leather).
Store the handgun in the holster at night. The leather, softened by a day of your body heat and sweat, will continue to break in overnight.
Wash the holster inside and out with a glycerin-based soap, if breaking it in is taking too long. Use only glycerin soap and water--this is based on the recommendation of every significant holster manufacturer, including Bianchi and Galco. Water temporarily softens the fibers, but will not permanently soften them as does oil.
Avoid soaking the holster, if you wash it. Only dampen the surfaces.
Wrap the handgun in a sandwich bag, and push it into the dampened holster for an hour.
Remove the handgun, and allow the holster to air dry, completely, before reuse.
A good-quality holster is made of top-grain cowhide, with a good finish; it will be very stiff, and fairly impervious to moisture. It may take some weeks before the holster feels natural. A hard plastic holster (for example, of Kydex) will not break in at all.