Greater confidence leads to stronger performances. There is a great difference in the confidence of a runner who approaches the start of a marathon having done the bare minimum training and that of the runner who has trained with variety, intensity, creativity, and persistence (and supplemented that training with a healthy diet, careful recovery, and mental preparation).

As a concert approaches, vary your study approach, take notes, and explore as much as possible to gain a better understanding of what yields the best results on stage and what makes you perform at your best. Depending on how well I know the music, I may start preparing months ahead of time or a few weeks from the date. Here is a checklist of actions that start to occur during the preparatory stage that are true confidence builders:

1. Warm-up with a focus on relaxation.
2. Befriend your metronome. Play all your pieces at a very slow tempo (for example, the Allegro from Barrios’ La Catedral at 1 sixteenth note/second, or a tremolo piece 1 note/second). I pick a quarter of the program and make sure that by the end of 2-3 days, I’ve practiced everything at a VERY slow tempo. Manuel Barrueco thinks slow-practice may be the best way to practice.
3. Build a list of troublesome passages or excerpts that do not feel effortless. Practice right hand and left hand alone going from very slow to as far beyond concert tempo for repetitions, practice in rhythms, analyze the space between notes, watch youtube videos of someone wonderful playing them to glean possible modifications, exhaust your resources to make the passages in question effortless. Before increasing tempo, try to nail 5 repetitions in a row. Keep a record of tempo and tally how many repetitions you’ve successfully done.  When focus fades, move on to a different excerpt or take a little brake.
4. Record yourself. I’m not a fan of this but sometimes you have to do what you have to do to improve. If you truly want to assess what you sound like, it is necessary.
5. Exit practice mode and enter performance mode by performing run-throughs. Play a run-through of each half of the concert at least once every two days over the course of the month preceding the actual concert. Two weeks from the performance, videotape the entire concert or perform it for a friend, a student, or colleague. Several full-length run-throughs always help to improve concentration. If you have time, experiment with performing run-throughs when you are tired, not warmed-up, or cranky. Perform it while the radio is on, perform it in the dark, etc… Learn to turn on your performance mode.
6. Break up your practice into several sessions a day. For example, instead of one large chunk of time (9AM – 1PM), try 9-11, another session 1-3, and then another 9-10. Don’t go more than 12-14 hours without touching that guitar.
7. Visualize the performance before falling asleep. Imagine the stage, imagine yourself on it, imagine every piece from the first note to the last. If you can’t ‘see’ what your fingers are doing, you probably do not know the piece as well as you think you do. If you have trouble visualizing, create a playlist of your favorite guitarist/s or of your best performance of the pieces and listen to it while you play along in your head. The music will serve as a bit of an anchor for your mind’s inner ear while you try to visualize along. Another technique is to watch someone play it and play along in your head.
8. Meditate often. A simple focus on your breathing for 5-10 minute periods throughout the day is a good start. I’ll write more on this in the future, but as a long time practitioner of yoga, pranayama, and other modes of meditation, meditation is one of a few things that always improves my day and centers my mind.

Good luck!