Easy. Just use it. Strap it on, shove a gun in there and be on your way.
No oils or what not; that will take the stiffness out of the leather and make it flop as well as making it not hold the gun as well.
“There are three reasons to own a gun. To protect yourself and your family, to hunt dangerous and delicious animals, and to keep the King of England out of your face.” - Krusty the Clown