There are various ways. One I like is to cut a pad of heavy rubber or plastic, maybe 1/4" thick, slightly smaller than the post. This is creates a nearly invisible shadow line under the post, and lifts it enough to avoid rot.
Steel connectors can be also be designed to provide a bit of vertical stand-off.
Another more common solution is to set the post on a square piece of pressure treated 2x8 as a plinth.
Any of these must be designed isolate the wood from the concrete and also avoid trapping rain, etc.
pete
