Apply a pH balanced leather cleaner to your couch, following the directions on the bottle. These products are designed to remove dirt and bacteria from the leather without damaging the surface, and they usually contain glycerin for adding softness to the leather.
Treat the surface with saddle soap. You can buy it from a horse and tack store, or make your own using the directions in the "Resources" section. Apply the saddle soap with a wet flannel cloth, working it into the surface with a circular motion. Polish until dry with another soft flannel cloth.
Spray white vinegar on the surface of the couch, which will kill odor-causing bacteria. Wipe away the excess with a clean cloth, and work a thin layer of oil into the surface to moisturize. Orange tree oil works well and has a fresh scent. Olive oil, neatsfoot oil and linseed oil also work well.