Leather Sofa Vaseline is something everyone has laying around the house and it's cheap. Dab a soft clean cloth into the gel and then work it into the leather sofa in small amounts until it feels dry. You will want to wipe the sofa with a dry cloth to remove any excess grease but the leather will absorb most of it.
Try this on a small area first before doing the whole couch.
Leather Sofa Pictures