Practice. I don't think you need to specifically practice concentration techniques personally. Practicing or playing for 5+ hours straight might be mentally tiring the first handful of times, but after you keep doing it you adapt, you don't get physically and mentally tired as much and concentration levels should still be pretty high. Also experience counts for a lot. With experiences comes the ability to figure out problems faster and with less effort. So rather than thinking about a particular layout, CB path, how much English and so on you instantly know what's required because you've hit similar shots, had similar layouts plenty of times before. This alone can massively help keep concentration levels high but with a combination of the two, it should massively help improve concentration.
What helped me was 14.1. When I first took it up I struggled making high runs not because of a problem with fundamentals, aiming, fitness and so on but because I'd get mentally tired, stop concentrating and then mess up. As I became more familiar with the game I'd recognize patterns instantly and wouldn't have to concentrate to progress through racks. It just flows.
Deep concentration may not be needed depending on what you play. For 9 ball its definitely not needed unless you're up against a world class player and 1 mistake can mean loss of several games. 9 ball is as simple as pool games come, learning a handful of patterns and CB paths is all you really need to be successful. So unless you're playing a 9 ball set for 6 or 7 hours, you can coast through without giving it much thought. 8 ball is a little different, safety play can be quite confusing at times and you do get in situations where you need to really think about what you're about to do. But still, after playing it for a few months you will get to a stage where you need less and less concentration when playing it. As for 1 pocket....i dunno. I've never given that game the time of day. Its dull, extremely slow paced and tends to cater to the older generation imo. So I couldn't possibly comment.