IMO once you have a solid foundation and understanding of the game and you are able to balance yourself, stroke smoothly and level, stay down, and know and be able to direct the cue ball and pocket balls well, this game becomes primarily a mental battle between you and yourself. I mentioned on one post elsewhere how much I liked the way the Monks book talked about the Id or inner you. Boy that sure a stopper for lots of us.
The catch 22 is you can't develop good concentration without first identifying it by experiencing it, and you can't experience it without understanding what it is. I'm no expert, but I think this is the key to the next level beyond merely advanced play.
To experience it in small individual chunks, that I call my mini zone and then putting them together into a dead stroke is what has to be practiced. The problem with practicing this is that it is not a tangable, physical thing to actually practice.
I've found this dead stroke a few times, and run high 70s, a few 50s. but for the most part I find I'm somehow able to wake myself up in the 20 to 40 ball area. Pisses me off actually