A good leather boot does not wear out; it dries out. The leather loses its oils to the weather, cracks along the flex point and lets the water in. Everything that follows is about keeping those oils in.
Brush the mud off dry, never wet. Rub a thin coat of wax polish into the leather with a cloth, leave it half an hour, then buff with a horsehair brush until it shines. Once a month in winter, less in summer.
Keep trees in them when they are off your feet, and if they get soaked, stuff them with newspaper and let them dry away from the radiator. Resole before the welt goes; our cobbler in Winchcombe does ours.