...for any part of the game (shot making, shape play, mental aspects, gambling...), and any source (books, DVDs, other players, forums...)?
Focus on the game your playing and pay little or no attention to anything that will distract you from doing so. When not shooting sit quietly and relax your body and mind, do not sit with a death grip on your cue, or with clenched teeth. If you do, when you return to the table you will bring this tension with you, and it will effect the fluid movement of your stroke. While waiting to shoot focus on your opponents game, and find their flaws and weakness, I don't care who we are talking about everyone has some weakness or some flaw with their game or they would never miss. While you are doing this clear your mind of negative thoughts and relax every muscle in your body, everyone has fear and everyone experiences situations that put them under pressure but how you use this energy makes all the difference.
Pressure can make you a winner or a loser it all depends whether it controls you or how you use it. Pressure can be used for strength as well as weakness, ever hear some one say fear is the mind killer well fear and pressure both can have that effect if they are left uncontrolled. However, if they are kept in check and controlled they can also keep you alert, and focused.
It all comes down to how you channel the energy, and this is done while you are quietly waiting for your next shot, this is how you prepare before you come to the table.
JIMO