Good leather needs very little. Most damage comes from doing too much, too often.
Every few weeks
Empty the bag and wipe it inside and out with a soft dry cloth. Check the zip and the feet for grit.
Twice a year
Work a pea-sized amount of neutral leather cream into the surface with a cloth, in small circles. Leave it for ten minutes and buff. Test on the base first: vegetable-tanned leather will darken slightly.
Rain is not the enemy. Heat is.
When it gets wet
Blot, do not rub. Stuff the bag with plain paper so it keeps its shape and let it dry at room temperature. Never use a hairdryer or a heater: fast drying makes leather stiff and can crack the finish.
Storage
Use the dust bag, keep it upright, and do not hang a bag by its handles for months at a time.