Clean the leather seats well with a damp cloth. If you need to break through the finish on a leather seat, you can use a little rubbing alcohol to help open up the grain of the leather. Test the alcohol on an inconspicuous area of the leather first. If the leather is dyed, the alcohol could cause it to bleed.
Apply a leather conditioner to the seats. Although this seems counterintuitive, the conditioner will actually help the leather soften, creating a more comfortable, relaxed and grippier finish. You may need to apply several coats of the conditioner over several days.
Spray a coat of leather sticky spray or apply pine resin to the leather if you need extra help. These products are often used on saddles to help keep riders in place.
Cover the leather furniture with a slipcover made of a nonslip fabric such as microsuede if you are unsatisfied with the seats. Microsuede or microfiber can often look like leather but has a softer, nonslip finish.