The first thing is to know when to focus. Humans can typically maintain very intense focus for about 12 seconds before the mind starts to wander. So a 9 ball game that lasts 5 minutes is too long to stay focused. The amount of time your bridge hand is actually on the table is usually around 8 to 12 seconds. So knowing when to focus allows you to do what you need to do when you are shooting. When you pick up your bridge hand, allow your mind to relax, and then start thinking about your next shot. Once you have made your decisions about the next shot, get down on the shot, and focus for another 8 to 12 seconds.
Short bursts of intense focus will allow you get the job done without wearing yourself out.
Steve