Like he said. I only use wooden posts when I can design an air gap between the post and the ground or concrete (even pressure treated wood). Even wooden posts mounted against a concrete retaining wall get a treated Cedar spacer.
If a post has to be wooden (for aesthetic purposes), I use square galvanized metal I.D. the same as the O.D. of my post and set that into the concrete at the base. And still leave an air gap between the wood and the ground (or concrete). Drill a few holes through the galvanized metal base for mounting and air and seal the top with acrylic. Galvanized metal in concrete is good for a long time, the high PH retards rust. Putting a stainless (or copper) cap/lid on the top of a wooden post also greatly improves longevity.
Nothing really beats good old 1 1/2" or 2 " galvinized water pipe for fence posts, set in concrete with a cap to keep the moisture out of the pipe (or a wad of newspaper and quick dry cement, painted after drying). You can drill, tap and thread small angle (I use stainless or chromate treatred angle) clips to the posts and mount fence elements between the posts to the angle clips. Makes it easy to dismount and retreat the wooden fence elements every 3-10 years (depending on climate). If an element gets damaged it is easy to replace. Ignoring the nails or staples already used to construct the wooden elements and installing screws before mounting the elements can greatly increase the lifespan. Wooden elements tend to spit out nails and staples as they age, screws stay put (though manufacturers may be using better nails and staples now). A little research into the size of the elements is helpful, every manufacturer tends to size a little differently, picking a firm that will still be around when it comes time to replace an element can save grief.
We get occasional tornados and/or hurrican force winds, I tend to over engineer my stuff a bit, but it lasts.
Galvinized metal, Hamerit non ferous metal primer (before painting the galvinzed steel if you choose) and Hamerite paint are your friends. A quality impregnating (that sinks in) and sealing (helps to shed water) coating for the wood, to be reapplied at intervals is also helpfull for longevity. Completely sealing the wood is to be avoided, wood needs to breath some. A little dab of anti sieze on the threads into metal makes later service much simpler.
A little extra for materials and an extra day of labor can triple the lifespan of your fence and make future repairs and service a whole lot easier.
Sorry about writing a book, but well built fences are a passion with me.
