semi-transparent stains will typically last 2-3 years, if you want something longer use a solid stain. Sikken Cetol-SRD and Cabots Clear Solutions are good semis. The sikkens only requires one coat, but the cabots has the option for 2 coats. The condition of your fence will dictate how you should prep it. I have no idea about additives, but I would be interested in learning about what you intend to do and why. If you pressure wash, I am assuming you are going to apply a stripper before hand? The first thing I would do would be to try and sand out the stain, to get a fresh surface for staining, with a finishing or palm sander. Maybe post a pic? let me know how it goes.