Empty the wallet once a month. Cards left in a stack bend the spine and stretch the stitching.
Wipe the outside with a dry cotton cloth. If the leather looks pale, use a pea of uncolored leather cream, then buff. Keep it out of a back pocket when you can. Sitting on it is what breaks most billfolds.
If it gets caught in rain, blot it and let it dry open at room temperature. Stuffing it with paper while it dries keeps the card slots from collapsing.