I'm of the belief that we create pressure by how we think or act, not by the situation we are in at the moment. The difference in playing a "friendly" game or an "important" game is not the game itself, because the game is always the same, but in the mental attitude of the player. I feel that a consistent approach to every game is the key!
The well known term in the pool world "Practice like you play" might be a bit off. If that term were more like "Physically and mentally practice like you play" it would be more beneficial as the combination of the two then become engrained and part of one's muscle memory. When the physical and mental work together it can become one; however, to be able to combine the two consistently is a tough nut to crack. Nobody plays pressure-free; however, combining the mental with the physical during preparation-engrainment can improve performance .. at least to "some" degree.
I "try" to maintain a proper mindset -- play the balls and not the opponent -- and when thoughts like "This is it!"; "If I miss this he's out'; "The whole season depends on this out"; "It is do or die", I use the same approach (muscle memory again) that I do when I'm down on a shot and things just don't feel/look right. I stand up, move back, walk around the table, take another look at the layout, breath deep, and regather myself. If after a minute I still have those thoughts -- it's time to call a player's time out.
Discipline and poise in practice become discipline and poise at the table.
<Now, if I could only consistently practice what I preach. Human nature is, for lack of another term, unnatural sometimes.>