Full-grain leather is meant to develop a patina over time — that's part of the appeal — but it still needs basic care to age well instead of just wearing out. The single most important habit is keeping it dry and out of direct, prolonged sunlight, which dries out natural oils and causes cracking.
For routine cleaning, a soft, slightly damp cloth is usually enough. Avoid baby wipes, alcohol-based cleaners, or anything with silicone — they can strip the leather's natural finish. If a spill happens, blot immediately rather than rubbing, and let it air dry away from direct heat.
Condition your leather bag every few months, more often in dry climates. A quality leather conditioner restores oils lost to everyday handling and helps prevent cracking at stress points like handles and corners. Apply a small amount, work it in with a soft cloth, and let it absorb overnight.
When you're not using a bag, stuff it lightly with acid-free paper (not newspaper) to help it hold its shape, and store it in a breathable dust bag rather than plastic, which traps moisture. Treated this way, a good leather bag should easily outlast its warranty — often by decades.