Box stores and smaller hardware stores such as Ace carry a post protectant that can be painted on the bottom 4-8 inches of every post. This protectant will protect against moisture created by morning dew and against bug infestations. As I tell many of our customers it should be applied every couple years and definitely at the same time a customer re- applies their stain,seal, or paint. If applied well it will greatly increase the life of any fence.
Very often we are called out to do repairs on a fence and most of the posts are breaking at the ground level. A repair cannot be done and at times a 7-10 year old fence needs to be replaced. That is due to base rot that can be prevented by this kind of maintenance. Base rot starts in fences that are only five years old. Its due to a lot less protection being added to fence posts these days due to the restrictions put on the industry. Let me know if you have any questions.