If you are just starting out to build, buy redwood or cedar as those woods will last the longest before rotting. If you buy thicker boards, they will last longer. If you aren't opposed to pressure treated lumber,it will increase the life span of a planter. Another tactic would be to line the inside of the box with a plastic or rubber liner to keep the wood away from the soil. For the exterior of the box, you can apply a natural oil such as linseed oil or an oil-based stain or varnish like one would use on a wooden deck. What you use depends on whether one is using the planter boxes for edible plants or ornamental flowers.