Spray new shoes with a waterproof protector. It will keep the surface from getting marred if they do get wet in the rain.
Add taps and half-soles of rubber to the bottoms. A shoe repairman can do this for around $20, and the reinforcements will add many years to the life of your shoes.
Alternate pairs. You want your shoes to have time to breathe between wearings. Also, let shoes dry for several hours before putting them in the dank recesses of your closet.
Clean the insides. This is especially important if odor is a problem. Swab them with alcohol or a drop of tea tree oil, an antifungal agent. Take care not to splash to avoid staining the leather.
Wear hose or sock liners. This will protects against perspiration, which erodes insoles.
Use shoe trees. They help retain the shoe's shape. "Use forms made of cedar for the pair worn that day to absorb moisture," says Joe Rocco from Jim's Shoe Repair in New York City. Plastic trees are fine for your other shoes. You can get shoe trees at shoe stores, shoe-repair shops, or at stores like The Container Store (plastic, $5 for 2, containerstore.com, or cedar, $20 for 2, containerstore.com).
Polish leather. The salt in sweat dries out leather over time; polishing regularly keeps it supple.