I think Bryan Kimsey sparked me to this topic with his analogy about archery. When I urged repetition, Bryan rightly replied that it's also important to hit it one time; I think his remark was that instead of shooting 100 arrows, it's also good to step up to the line and shoot ONE arrow. (Then he also said you have to have a lot of repetition to be able to do that.)
He's so right. It's not enough just to practice; you have to practice with concentration.
When I wrote earlier about repetition, my point was that you have to drill the notes and fingerings and phrasing and moves into your brain and muscles. And they will go in there with constant repetition. But that in itself is no guarantee that come out again on demand, especially when you're under pressure. As an example, I could cite a tune I know in my sleep, like "Red-Haired Boy," which is drilled so far into my cortex that it'll probably never come out. But I've had times when I've tried to play it in public where it was nowhere to be found--you'd have thought I had never even heard it. And if this could happen to one of my old stand-bys, then what about something new?
So what does this have to do with practice? Everything, I think. At some point , you need to stage for yourself something like "dress rehearsals." When you're preparing something to play in public, and you've gotten it far past the learning stage, you need to mentally shift gears from the learning mode and go into the performing mode. Visualize yourself on stage or at a jam, ready to play the tune or piece. Relax yourself if this image makes you tense up, remembering that tension is your enemy (too much tension, anyway; you probably can't play without some). Breathe away your tension, then play the piece. One time. Perfectly. (Yeah, right.)
Okay, it wasn't perfect. But don't stop. Play on through. Before, when you were learning, you had to stop and isolate trouble spots and work on them. Now , you need to push on through, even if you mess up. Then note the places that give you trouble so you can work on them. Think of a dress rehearsal for a play . Rather than stop and work on individual scenes, the actors do the whole play, pretending it's real, even down to wearing costumes. Even if they mess up, they push on. Then the director gives them "notes," pointing out the places that need work. That's what you have to do for yourself--then you can go back and apply the practice skills you've built up to trouble spots. I've used this visualizing with good results. You can even do this without your instrument, by the way. I've done it in the car on the way to a job or jam. You're gonna have stress and tension when it's "real," but visualizing can help you overcome that.
You can also concentrate in ways that are counter-productive. I read something about mountain biking once that applies here, I think. It was talking about avoiding obstacles, and it said if you look at an object, you'll hit it. (It's true, too, which a trip over the handlebars showed me.) The same is true with your obstacles as a musician. You can concentrate so much on them that you fixate on them and you'll hit them every time. That tricky place in a tune is like a big rock or tree trunk in your way. If you keep staring at it, you'll hit it and mess up every time. But once you can step up to the line and shoot that arrow (back to the archery!), you can do it every time. Positive concentration is what gets you there.