Just joking, not a secret at all. We used to cut a piece of a broken post about 3-4" long, drill and screw it onto the bottom of the post to make a sort of upward-curving hook. Those are pretty effective - if set deep enough, and the dirt is tamped down around, you cannot pull it out - it has to be dug out. The downside is you have to dig a hole to set the post in - can't just pound it in with a mallet.
But then a park service guy came up with a better way - cheaper, faster, lasts longer. He just heats the bottom of the post with blow-torch and bends it up into a hook.
