Leather conditioner.
Tanner's Preserves or Lexol are 2 good ones.
Hit them every couple of months. Maybe more in the summer or depending on if the car sits outside all the time...
Whatever you do: DO NOT USE ARMORALL on them or any other product made for vinyl, rubber, plastic, or dashboards. This will cause the leather to shrink and crack.
Only use a leather conditioner.
Also those "leather wipes" are only good for spot cleaning the leather. They do not supply enough real conditioner to be more useful then a cleaning wipe.
