Clean leather bags with clear shoe polish wipes -- the oils are moisturizing and much better than water. Keep the leather clean by avoiding handling your bag after applying beauty products, such as face foundations or hand creams.
If your bag has a chain handle, store the chain inside the bag to avoid scratching the exterior leather when the bag is resting.
Prevent color transfer by consciously resisting the urge to rest your bag against your jeans.
Stuff smaller and softer totes with tissue paper to avoid losing shape, and store them in dust covers or natural fiber pillowcases to protect the exterior.