Clean the leather furniture thoroughly using a professional grade leather cleaner. If possible, have the furniture professionally cleaned.
Treat the furniture with a leather conditioner. Pastes and oils tend to last longer than spray-on conditioners. Reapply conditioner on a regular basis. If the garage does not go through significant temperature or moisture variations, twice a year should suffice. Increase frequency based on storage conditions.
Place the furniture away from external walls, especially if they are not insulated. Leather can be damaged by significant temperature and humidity fluctuations.