The other day after I refinished a door for a new customer they asked me “How do I properly maintain my wood entry door?”
My response was you must beware of the environmental abuse the wood door takes from the elements (sun, water, and dirt) to keep the finish as long as possible. Here in Denver we are a mile closer to the sun where the ultra-violet light is more intense than other parts of the country. This intensity of light or heat causes a larger range of temperatures on the door, stressing the coating with heat during the day and cooling off at night. This expansion and contraction is a major contributor to in the breakdown of the protective clear coat.
Another thing you want to consider is any water that sets on the wood door will create problems too. Melting snow or just water in general is usually a major culprit that finds a way past the protective coating. Usually a really cold snow is not the problem; it is the melting which brings water to penetrate the weak spots in the coating. Oh, and keep the sprinklers off the door during the irrigation months.
So the last thing I want to mention is the dirt that finds its way to the door should be removed periodically too. The dirt is abrasive to the finish and needs to be removed with a moist sponge a couple of times a year. Just like your fine wood furniture, your stained wood door needs to be cleaned to help avoid any film of dust.