Hi Baxter
As we usually (mostly

) all now, pool-players know everything-including every single excuse for everything (kidding, ironic and sad but true^^).
The mental part is for sure a critical point in billiards and important, too. Back to your main *theme/question*. If we are talking about the mental stamina, we re talking about pyschologic. Mental training is a very huge field which includes many many parts.
Of course you may get some ideas or advices from friends, advanced players, instructors or professionals-- but it is really necessary that a person has some knowledge about this physchological things.
For example if you WOULD decide to work with a knowledged (professional?) mental coach, he would NEVER give you a single thing to work on, or to learn. Just if we would talk about meditation or relaxation techniques, we again jump into a field which is terrible big area. The knowledged mental coach would start with offering you several different relaxation/meditation methods, because there s no *one methods fits for all*.
If you re really interested in stuff like that, and also want to *dive a bit deeper* into these things i recommend for example books from Nideffer- a book which is great is called *psyched to win*. You can find a lot of stuff about nideffer also via google.
Mental stamina itself can be reached also by standing at the table- with several drills. but every human is individual- so it s for sure not that easy
lg
Ingo