A couple months ago the WSJ had an article that analyzed his messaging technique. They showed how he will choose a theme, test it out in a smaller audience (for example, the speeches he gives as he crosses the country and most people ignore). If the audiences respond well to it, then he tries it again for a larger audience. If they don't respond well, then he drops it.

That, of course, in addition to his several decades polishing his messaging techniques in the public eye (some people would call them click-bait techniques, but whatever).