Do not store a gun in the holster.
At the end of the day when you remove the gun from the holster, wipe the gun down with a very lightly oiled cloth- just enough to remove things like finger prints. The bluing should have like a skim coat of oil....just enough so you can barely feel it if your fingers touched the metal. It should NOT be dripping with oil.
Every 6 months or so give the leather holster a light cleaning with saddle soap or some other product specifically designed to clean leather then rub in a small amount of beeswax, snowseal, or neatsfoot oil to lubricate the leather fibers. Any product that is made for leather should do the trick, however if you use neatsfoot oil, make sure it is "100% pure neatsfoot oil". This product is somewhat hard to find(try tack shops), you will see a lot of times a bottle marked "neatsfoot oil" at the hardware store but if you check the bottle you will see that it is actually a petrolium product- do not use this kind as it will damage your leather and stitching over time.