I thought I'd pass something along, since it comes up fairly often. Breaking in a tight holster. Following the recommendations of several holster makers, this is what they suggest for leather. First and foremost, double check that the gun is unloaded....

1. Slightly dampen the inside of the holster by wetting your fingers and working it into the leather. The key here is to lightly dampen the leather.

2. Put the gun in a plastic bag, like a gallon size Zip Lock.

3. With the pistol / revolver in the bag, holster and unholster it a few times to loosen the fit. Then leave the gun in the holster a few hours until it has dried.

** For holsters that are overly tight, you can use two bags, for added thickness**

Exactly. 
New at this: you can do it twice or three times.  You can never make one tighter after too much break in.

A Couple of Better Ways:
• If the holster is not extremely tight, merely practicing your presentation a whole bunch of times will do the trick pretty quickly. This is the method that the holster maker would prefer you to use.
• If the holster is terribly tight and you don't want to practice-draw that many times, use a teflon-bearing leather lubricant like KG-9 on the inside of the holster. Beware, because this may work all too well, and it takes a while to wear off.
• Rubbing alcohol is better than mere water for stretching leather. Use just enough to dampen the inside surface, and never use more than one layer of plastic bag. Alcohol evaporates faster than water does, so the job is done sooner. Alcohol will preserve the stiffness of the leather better than mere water, too.

Never, ever, use saddlesoap or neatsfoot oil on a good holster. Avoid Lexol, too, if you can. All of these preparations will soften the leather and defeat the intended purpose of a wood-hard holster.
If you have to use something, use only neutral (colorless) cake shoe wax ("shoe polish"), or even cake furniture wax. Use it sparingly, because too much wax can also soften the leather.