Wet the furniture piece thoroughly with a large bath sponge until the item is completely soaked through. If possible, it is better to do this somewhere outside within safe distance of a power supply.
Dry the piece quickly with a blow-dryer at full heat. According to leather information site Ally Cat Scratch, the drying process will produce the "cracked" look of old leather. Once the piece is dry repeat until you have the age you want.
Apply a coat of leather balm (a microcrystalline wax in an alcohol solution) to places on the furniture which would naturally receive the most wear and tear (for example, the arms of a couch). Apply the balm thickly with a rag and then quickly and roughly brush it off. The balm will give the piece an oily darker look in keeping with heavy wear.
Sand parts of your piece which would have been exposed to the sun over the years to give those areas a bleached appearance. Ally Cat Scratch, however, only recommends this if you want a very distressed look. Rub a little at a time; it is easy to distress a piece but difficult to return it to its former condition.
Bend, twist and beat your leather once you have the look you want to break down its fibers and give your piece the feel of something worn in.
Drill small holes into any wooden parts of the furniture to mimic woodworm. The website Stitch n Save recommends using different-sized drill bits at different angles to make the look more realistic.