Apply indirect heat to the surface of the leather couch with a hair dryer to give it a "sunbaked" appearance. Start on the lowest setting, and work your way up to higher heat. This will prevent accidentally singeing the leather.
Expose your leather couch to the elements. Move it outside in the summer sun for several days, which will darken the material. Note that this method of aging the leather works best on vegetable-tanned leather, not chrome-tanned leather. Check with the manufacturer of your couch to verify how the leather was tanned.
Rub the couch with sandpaper. This will distress the leather, and make it look worn. You can rub the couch all over, or rub selected areas like the cushions, armrests or back.
Hit your couch repeatedly with a belt that has a metal buckle. The metal surface of the buckle will cause minor damage to the leather, creating dents and scuffs. Be careful not to hit the leather too hard, or you could tear it.