The way I prepare for high pressure shots begins long before the shot itself.
I am a firm believer in developing a pre-shot routine that is the same for every shot. Through Practice, Practice, Practice, it becomes second nature. When the pressure is on you can focus on the steps you execute to set up and take every shot.
The best way to develop a reliable and consistent pre shot routine is to work with a coach or instructor. If you try to do it yourself you could develop bad habits that will hold you back.
My pre-shot routine has evolved over the years. At this time it includes, Do my thinking/planning standing up. Aim the shot standing up. I go into my stance the same way every time like a one two dance step. I set up as if shooting the shot one handed and aim it as such before my bridge hand ever touches the table. Once my bridge is set I am sighting and stroking to confirm that striking the cue ball where I planned is going to deliver it to the object ball the way I want. Then I take a pause at the cue ball, a pause at the back then a pause at the finish of the stroke.
If I miss a shot I expect to make, it can usually be attributed to a breakdown in my routine. At the same time by following my routine I can make shots that I am not comfortable with until they are in the pocket.
I started a thread back in 2013 in this forum titled,
Critique my pressure match.
It included a link to a match I played in the Spokane Open that year.
My highlights at Spokane Open 2013
Watching the first rack I am shooting, I made some shots that were hard for me. When I got to the nine ball I had a breakdown in my routine(my right hand was 6 inches out instead of tight to my chest in the set up.) which led to a pathetic miss on the nine. I did get lucky and got a second chance at that same nine. It was another hard shot for me but that time I followed my routine and pocketed the ball.
While my routine has changed from what it was then the most important factor is having and following the same routine every shot.