One tip I have on concentration is, “Concentrate on the goal, not the method used to reach the goal.” For example, it you are making putt in golf, concentrate on the hole and the ball going into the hole, not on making the putt itself. Don’t concentrate on the putting method since that will cause you to think about too many factors and cause you to tense your body. The putting itself should happen unconsciously due to your previous intense putting practice. It’s similar to driving a car and having a child run in front of the car. You do not think about braking or how much to brake, you just keep staring at the child and your foot applies the brakes reflexively due to all your experience in braking. Off-road motorcyclists know not to look at obstacles; if they do, they know they will probably hit them. They look where they want to go and let their training take over obstacle avoidance. If you run off the edge of the road with your car, experts tell you to keep looking down the road and let your driving experience steer you back into your lane. If you look where the car is going at the moment (off the road) and concentrate on that, then that is the direction you will probably go.
Therefore, while playing squash, relax and concentrate on where you want the ball to go, not on the methods used to get it there. If you have trained enough, your body will use the right method without your conscious thought.