Don't go too far in the other direction!
Blackjack nails this in his book. But basically never let down until the last ball is sunk. I had to learn this as I would get lazy on one or two shots and give up matches. Most of the time I shoot every ball like it's the last ball. When it comes right down to it, every ball is a money ball.
In fact I can't even tell you the score during the match until they stop me and tell me it's over. I don't want to know the score because it does not matter until it's over.
It's easy to get down on yourself during a match but it's also just as easy to get over excited when you get on the hill. Keep an even keel in both directions.
This might seem simplistic but it works for me.
Great advice in that bolded paragraph, but beware -- don't overdo the "shoot every ball like it's the last ball" thing. That may work in rotation games like 9-/10-ball, but can put you in an awkward situation in ball-tally games like One Pocket and Straight Pool. Personally, I have the habit of losing track in the match. Like that scene in the original Matrix movie, I take that red pill, and I spiral down the rabbit hole / get so focused during a match, that I lose track of how many balls I sunk into my pocket (in One Pocket), or how many balls I've sunk in my run / total point tally (in Straight Pool).
It happens with me a lot -- I get so focused and so, well, "tunnel visioned," that, in One Pocket, I'll often sink 12 balls in my hole, before suddenly realizing that I'd already won the game 4 balls ago. And I'll look up from the table, over to my opponent in the chair, and he's got that sh*t-eating grin, laughing at me. He'll say something like, "I was going to let you go and see when you finally pulled your head out of your @ss, when you finally realized the game was over long ago." He's right, I look like a complete fool, and I shrink back to my chair, wanting to hide my face. Very awkward!
The same thing happens in straight pool -- I'll lose track of how many balls I've sunk in my run, and I've a hard time recalling -- having to count the balls still left on the table, etc. I'll get laser-focused, running balls, before my opponent will [either coyly or innocently] ask "hey, what's the score?" And then I'll realize I overshot match point by like 10 balls. But this is not nearly as embarrassing as the One Pocket faux pas described in the paragraph above.
To deal with my personal "lost in a rabbit hole" issue, I'm thinking about getting one of those thumb clickers (talliers?) to clip onto my belt, that I integrate into my post-shot routine to click it with my thumb to tally another ball, the tally of which I periodically look at to remind myself of the score. But that's my personal issue.
The real point here is not to go too far extreme, focusing so hard that you lose track of other important things around you, as well as that 20,000 foot overview of the game itself (i.e. the score!). It indeed is a careful balancing act, and I think we all have a little bit of "extremist" in all of us.
Hope this helps,
-Sean