When rain and the elements have an "open door" in, the rot and decay process are much faster than if you keep the doors shut. This is an easy precaution that can save you money down the road. This goes for your windows too. 

One of your greatest enemies... Watch for the slow drip. If you find a leak, try to get it fixed as soon as possible. Replace bad metal and button down the pieces that are flapping in the wind. This will also prevent some unwanted creatures from coming inside. 

If you don't have metal siding on your barn, keep an eye out for the rotten boards that need repaired. Then adding an extra layer of protection by applying stain or paint to the outside of your barn slows down the decay process of the wood and creates a nice look at the same time. 

Since grass and vegetation have a lot of moisture in it, trimming it back will help moisture such as mold, from rotting your barn.