Lately I have put great emphasis on slow practice and the use of the metronome. I would like to outline a few of the benefits of slow practice.
Rhythmic Accuracy
Metronome settings that are significantly slower than performance speed consistently demonstrate a player's tendency to rush or drag. Many of these tendencies linger for years and cannot be solved by mental effort alone.
Quality of Sound
Slowing down the music you play allows you to truly listen to the kind of sound you are making. As you take the time to gradually speed it up you will notice when the sound quality begins to suffer. Concentrate on the sound first, then on what is happening physically and mentally to create that sound. Experiment. The practice room is a lab and if your hypotheses are not always correct and your experiments fail it does not mean that you are personally a failure. In fact every failed experiment brings you closer to your goals.
Preparing Motions
A major part of slowing a passage down is to figure out where the "difficult" parts are that trip you up in performance. As a performer, I have found that these moments are usually areas where I'm uncertain about how to prepare the motion. You may find that it is a difficult shift or an awkward bowing or string crossing that is causing the trouble. Slowing it down will help you to understand how that movement should be executed as you gradually bring it into real performance tempo. Also the repetition reinforces the movement.
Eliminating stress
As you practice slowly observe the things you do that serve no real purpose in the music making. Are your shoulders raised? Are your feet curled up or sticking out? What about facial expressions? Are they voluntary? Slow practice will allow you to identify when you make these extra movements. When you have identified them you can practice a passage slowly concentrating mainly on keeping that body part relaxed. For example, my tendency is to tense my mouth, either by an upper lip snarl or by tightly pursing the lips or clenching the teeth. As I play a passage that is normally quick at a slower tempo I will see exactly the shift, the fingering, the point of articulation that causes me to tense up. In slowing it down I can prove to myself that it can be played with a relaxed face. And as I speed up the tempo again I reinforce that it can be done even at performance tempo.
In short, make friends with the metronome. Frustration tends to come from the disappointment of not being immediately succesful. Slow it down and see it for what it really is. Every error in playing is a symptom of an underlying problem that is often invisible at the surface. Take the time to slow it down and discover it.
