1) Always start up your practice session with warming up. This piece of advice is aimed toward wind players, not strings players. Long tones are a great way to start up your practice sessions; it'll get the air moving and get your brain focused on listening to your tone. Don't rush through the warm up process. Warming up should take more than half of your practice session. As you play through your long tones, think about your tone. Is it airy? Strained? Unfocused? Judge your tone and improve it whenever you can.
2) Practice in front of a mirror. Check your position all of the time. For flute players, look at your hand position and check to see if your fingers are curved and if your fingertips are on the keys, not the middle of your fingers. If you have this issue and you can correct it, you'll be able to play faster and more accurate. Also check to make sure your fingers aren't flying off the keys too high. If you keep your fingers closer to the key, you'll be able to play faster. For violinists, I recommend watching your bow in the mirror and constantly checking if it's straight. This is an issue for several beginner string players. Note: If your elbow is going behind you as you play the violin, then your bow is probably not straight. Also while you're looking in the mirror, check your left hand position and make sure that you're keeping the heel of your hand off the neck of the violin.
3) Take some time and work on scales, scalar patterns, and arpeggios. These are the meat to tons of musical literature. Learn all your major scales first and then move onto minor scales and then all forms of the minor and so forth.
4) As you practice, continue to think about your tone and ask yourself, "Is my tone consistent throughout a piece of music?" Make that call and fix it.
5) Practice with a metronome! It'll become your best friend and you'll use it throughout your musical life. This brings me to another point.
6) Practice SLOWLY and then gradually speed it up after time. If a piece of music is giving you a lot of grief, practice with a metronome at a ridiculously slow tempo to ensure you're playing all of the correct notes and correct rhythms. Once you're completely comfortable with it at a slower tempo, speed it up a few clicks of the metronome. Assume you practice it at tempo 60 and it needs to be at tempo 80. Speed it up to 63 when you're ready and then move on to 66 and so forth. Before you know it, you'll be at performance tempo, and you'll hopefully be playing everything correctly if you've followed my advice so far. If you're having trouble getting the rhythm correct, try putting the instrument down and clapping the rhythm. Sometimes, if we take away the pitches and the mechanics of playing the instrument, the musician can focus on just the rhythm. It also helps me to try singing the rhythm.
7) Learn some technical etudes. Every instrument has a book of great etudes to play through. Right now on the flute, I'm working on Berbiguier etudes and on the violin, I'm working on Wohlfahrt etudes. Your skill level will dictate what book of etudes you'll be working on.
8) Sometimes singing the music out loud helps with getting the style and articulation right. Try it out. While you're singing also think about dynamics.
9) If you have a particularly difficult passage of music to learn, don't try to conquer the entire piece in one run-through. Try chunking the music into pieces and learn a couple measures at a time. Breaking the composition down into bite sized pieces will make life feel loads easier than trying to swallow the entire thing whole. It's often recommended to figure out where the most difficult measures of music are and then start learning them first.
10) If you make a mistake, fix it. Don't keep playing it wrong day after day. Remember: practice makes permanent. The way you practice is the way you'll perform.
11) Take breaks every now and then in your practice session. Let your hands and fingers rest before woodshedding another segment of music. Should you ever get truly frustrated in your practice session, please do take a break. Nothing is worse than practicing when you're mad at your instrument or your fingers.
12) Practice means a lot of repetition, so don't be annoyed by having to play material over and over again. When you practice, don't just practice until you get it right; practice until you can't get it wrong.
Good luck to anyone who is inspired to start practicing after reading this! I hope this helps!