In the Store: Shop for shoes at the end of the day, when your feet are a bit swollen (like they will be on the trail).
Wear hiking socks when you try on shoes.
Good stores will have an incline board that you can stand and jump on to ensure that your feet won't slide forward on descents.
In the Field: Take your shoes off and air your feet during long rest stops. Keeping your dogs dry helps prevent blisters.
Use aftermarket insoles (from Shock Doctor, New Balance, Superfeet, and others) to increase support and all-day comfort.
At Home: Leather is skin: It will dry out and crack if not treated. Clean thoroughly and apply leather-conditioning agents from Nikwax or Granger's.
Shoes soaked? Stuff with newspaper to speed drying.