Wear the jacket as often as possible. Yes, it seems like common sense, but the more often you wear the jacket, the faster it will age. Outdoor events are particularly helpful, as sun, wind and blowing dust assault the leather and wear the fibers.
Wet the jacket with a spray bottle full of water and apply a hearty coating of saddle soap. Rub the bristles of a stiff brush over the soap and scrub the leather vigorously. Move the brush in tight circles, pressing down firmly to soften and wear the leather. Squirt the jacket with more water to rinse.
Rub the wet leather with fine-grit sandpaper. Yes, sandpaper. The rough texture simulates years of abuse by the elements and removes the shiny, glossy finish from new leather. Sand every exterior surface of the jacket and hang it over the back of a chair to dry.
Butter up worn-out leather with leather balm. Dampen a soft cloth with a little water and rub the rag over the balm to produce a lather. Massage the balm into the leather and let it dry. The jacket will look streaky and waxy, but once the balm is dry, buff away the excess with a clean cloth to reveal the soft, aged leather below.