There's one other thing I've seen suggested, which is to actually work out your eye muscles used for focusing. I think I've heard this suggested in the past, but I saw this detailed in an old copy of "American Handgunner" that someone had left lying around my office (Note that I don't approve of this kind of thing myself. Being English I'm firmly against allowing rebellious colonials to own weapons, who knows what kind of trouble they'd get into ;-)
Anyhoo in the 2004 Sept/Oct edition, page 44,
there's an article quoting Ernie Langdon advocating eye exersises. The one that sounded relevant to Pool playing was to hold a pencil (Or other small object) in front of your face as close as you could to still be able to focus on it. Then you should switch your focus to a more distant object and repeat.
It seems to me if you setup to do this with a cue ball and an object set at the maximum length of shot you would find on a 9' pool table and practicied this a few times a day it may help in long sessions and tournaments.