Two things work pretty good, as long as the leather isn't too far gone. Dry rot for example cant be fixed.
Neatsfoot oil, applied sparingly, a very thin coat at a time and let it soak in before you apply the next. Please note that you want Pure Neatsfoot Oil, not the Neatsfoot oil compound. The compound has extra petroleum distillates and will harm your leather in the long run.
The other is to get a beeswax balm, a mix of beeswax and oils that will help to condition and apply a water repellant surface as well. Sno-Seal is a good one, along with Aussie Leather Conditioner. Spread it on the leather evenly then let it soak in or speed it up a bit with a hair dryer. Like the neatsfoot it is possible to over do it, so apply it sparingly and give it time between coats.