Neutral shoe polish or a floor wax (I love Johnson's Past Wax) is a good way to protect it.  I've been using Obenauf's LP oil as it doesn't soften the leather like a lot of products do, but will keep the leather from drying out and cracking.  It's also good for leather straps on shoulder rigs.  It keeps them from hardening and cracking.  Use sparingly of course, don't soak your holster every week with the stuff. An application or two of neutral or paste afterwards will be very helpful also. Stay far far away from mink or neatsfoot oil.  Those will turn your formed holster into a soft mess, and can actually cause the leather to break down.  Too much use of saddle soap will do the same, but it will take many applications before it happens.  As long as you only use it when needed you'll be fine.  Don't clean your holster every weekend.
Wax based products are the best bet for anything leather. They seal and protect, but don't cause the leather to soften anymore than it does from use.