Avoid spending the majority of your practice only on the concert pieces you are currently learning. Make sure to practice and isolate techniques that you will need to become a better musician in general. Playing correct notes and rhythms are only two parts of any successful performance. Don't forget to focus on:
Tone - A good tone is the #1 fundamental to build any successful performance on. Practice long tones on open strings and focus on bow placement, weight and speed.
Intonation - Pay along with a drone or recording, drill finger patterns and scales, isolate short sections of your piece and listen carefully to your intonation.
Articulation (Bowing Style) and Dynamics - Drill multiple different bowing styles each time you practice, practice excerpts with and without slurs, isolate and practice the dynamic changes you want to make in your piece.
Hand / Finger Dexterity - effortless left hand finger motion is a key to playing more advanced music. Drill finger patterns, scales, and shifting exercises.
Vibrato - Drill vibrato exercises daily when possible, isolate places in your piece where you would like to add vibrato.
Practicing should not just consist of playing your pieces over and over. In order to meaningfully improve at something, make sure you are using the following practice techniques:
Tempo - Play things at a variety of speeds. Learn your music slower than the correct tempo and faster than the correct tempo so that you are flexible to play at any speed. Try every new technique you learn slowly at first then gradually speed it up.
Repetition - Many of the things we learn on an instrument require "muscle memory" to perform correctly time and time again. Practicing specific, small selections of music and techniques repeatedly develops the muscle memory for you to perform that music or skill successfully in the future. For instrumentalists, it's true that "if you don't use it, you lose it." Musicians have to continue to drill their skills regularly to stay "in shape."
Isolation - An efficient musician must be able to first identify a problem they might be having then isolate that problem to practice on it. On a difficult piece of music, isolate a few notes at a time and gradually add more. When learning a playing technique, isolate the fundamental movements or actions that create that skill to discover which aspect of the skill is giving you trouble. You be the detective! Break down the individual motions or elements of a skill to uncover the most specific aspect that is giving you trouble.
Hand Independence - Always warm-up and drill certain techniques by isolating the right and left hands/arms. Warm up with long tones on open strings to focus solely on your bow arm motion. Warm up your vibrato, shifting, finger patterns, etc. at times without sound to isolate only your left hand/arm motion.
Fun and Exploratory Discovery - Take some time whenever possible to try things in your practice "just for fun" - things you are not specifically working on for a performance. Try out a more advanced technique for the fun of it, sightread a new piece of music, try to learn a song you know by ear, make up new sounds on your instrument, improvise something off the top of your head or compose something for yourself. Explore the capabilities of your instrument, your body and your mind and you may discover an even deeper passion for making music.
The amount of time spent practicing will depend on age/maturity, your goals, and sometimes on your schedule. Studies have shown over and over that consistent practice (daily) is much more effective than binge (all at once) practicing. We recommend practice sessions somewhere between 25 and 45 minutes at a time. If more practice is desired (or required), take a break first and come back to it with a relaxed mind.
Here is a sample practice routine for a beginner:
Warm-Up: 1-3 minutes
Play a fun, familiar piece: 3-5 minutes
Work on a new or difficult piece: 5-10 minutes
Work on technical requirements (scales, etudes, exercises, etc.): 5-10 minutes
Play something fun to end your practice session
When learning a fast passage of music always practice slowly first. Focus on playing the correct notes well in tune. Do not speed up the passage until you can play it accurately at a slower tempo. Focus carefully on intonation, many students play fast passages very out of tune because they never take the time to slow it down and focus on intonation. When you are close to performance tempo, try the "add a note" method - drill only two notes at performance tempo, then add one more, then another, etc. to build your excerpt.
When learning new music work on small sections at a time, even just two notes at a time. Try not to get overwhelmed with the big picture. Every piece of music is just a collection of notes, rests and other symbols plus the emotion and musicality you will bring to it. You can learn the notes one at a time, like piecing together a puzzle. Practice small sections then larger sections and eventually you will have the entire piece learned.
When learning a difficult passage, try practicing slowly through the whole part and then try the "add-a-note" method. Play the passage at the desired tempo but add only one note at a time. For example, practice the first two notes of the passage at the correct speed for the piece. When you can play those correctly add the third. When you can play the first three notes correctly add the fourth and so on...
Try not to get frustrated with yourself if you aren't successful with something right away. You won't get the most out of your practice time if you are getting anxious and frustrated with yourself and focusing on negative things. You need your brain at peak performance level just as much as your body to make most efficient use of your practice time. Try taking a short break and when you feel more calm continue practicing.
Always practice with good posture and instrument position. If you practice with bad posture or position you will train your fingers to play in tune when you are playing with bad position. Then what happens on a performance when you are trying to play with correct position? You play out of tune!
If you are able to, practice in front of a mirror at home. Play something that you have memorized, like a scale, and study your instrument position, left hand and right hand position while you are playing. Better playing position contributes to better playing ability. Try to notice any areas in your position that could be better. Concentrate specifically on anything your conductor or your private teacher has asked you to work on.
There's a time for running through a piece beginning to end and a time to stop and work on small sections whenever you make a mistake. In the beginning stages of learning a new piece of music, stop when you need to and correct the mistake. When you are getting close to a performance date, you must practice running through the piece without stopping for anything, even if you make a mistake. You can't stop in the middle of a performance or audition so make sure you have practiced playing straight through without stopping. It takes mental practice to keep going and not let a mistake ruin your concentration.
When preparing for an audition, it is important to try all the tips listed above and also practice how you perform under the conditions of an audition scenario. Often in auditions there is a multi-person panel listening to you. Try getting together a small group of friends or family to listen to you play a mock audition. Find out as much as you can about the audition process - location, room, order of excerpts to be heard, number of rounds taking place and possible break or wait times in between, etc. to be mentally prepared for what you will experience at the audition site. Listen to as many different recordings you can of the pieces you are auditioning on to present the most professional interpretation of the pieces possible.
Practice with a metronome.
Start Slow! Get it clean, then speed it up.
Chunk it out. Isolate problem passages.
Swing it. Then reverse swing it.
Play it backwards.
Run a speed round.
Set a timer for 2 minutes. Check posture, grip, etc.
Practice in front of a mirror.
Roll a dice to see how many times to play.