Here's what Gary Brommeland recommends - I just got a whole rig from him and doing it this way works.
Carefully unload the weapon and wrap it with 2 layers of wax paper (with the wax to the outside), and seat it fully into the holster. Let it sit overnight. Next, work the weapon in and out of the holster a few dozen times. Remove the paper and try it. If it is still too tight, then repeat the process with 3 layers of wax paper. Continue until you get it close to the desired fit (be sure not to overdo it - it is rather difficult to un-stretch leather). Discard the paper and do a few dozen practice draws. The holster should now be sufficiently broken in to allow you to wear it and finalize the break in process. This is done by repeatedly drawing the weapon while the holster is actually being worn. Be advised that a properly broken-in holster should retain the weapon during vigorous physical activity, yet still release the firearm for a smooth draw.
Another approach is to layer thicknesses of plastic bag around the gun in the holster instead of wax paper, though wax paper is somewhat easier to "adjust" as regards thickness, is easier to come by, and is perhaps easier to work with. I've used both materials for some VERY TIGHT mag pouches and guns, and I prefer the wax paper method myself.