I often say as a joke that if someone's evening is ruined by me missing a note or two in the concert, I'll gladly reimburse his ticket!
Seriously, I use to be upset after missing a note in a concert or even in rehearsal. With time, I learned to accept that I'm not a machine and that missed notes are just part of the show. Of course, no one misses notes on purpose but if you just focus on that, you get more nervous about it and end up missing more.
There are a few things you can do to improve your accuracy though. The most important is to hear the notes clearly in your head before you play them. Try singing your music with your voice before playing. Take your ear training class seriously! We often say, if you can sing it, you can play it.
Being in good shape and having a good feeling in your chops will also help you. If you get tense, you'll miss more notes. That's why I said earlier that you shouldn't worry about it too much. You'll end up in a vicious circle of worrying-getting stiff-missing more-worrying more etc.
Be patient when you learn a new piece. Your brain and body need time to develop the right reflexes for each new piece or etude you play. If you allow yourself proper time and don't expect perfection immediately, you'll improve gradually and will gradually miss fewer notes.
When I go to a concert, I want to hear great music, musicality and energy from the players. I don't really care if they miss a bit as long as they try to make something interesting. I tell my students not to try making something perfect but something beautiful. The purpose of music is not to hit all the notes but to touch people's hearts.
Next time you practice, free yourself from the obligation of hitting all the notes. You'll still miss a couple but you'll sound much better, your chops will feel great and you'll have a lot more fun playing.
When you produce a tone with your instrument, you are not only using your lips but also all the muscles involved in breathing. You need to make sure that you coordinate all these muscles with your lips properly to get the best possible result.
One comment we hear about a singer sometimes is that "His tone really comes from deep". That's because they need to use their entire torso to move the air through their vocal cords to produce a nice tone. We can learn from that and learn to support our chops with a good air stream.
It's not very complicated after all. Here is something you can do to practice your air support:
1) Stand up or sit with your back straight.
2) Fill your lungs with air completely and let the air come out without making any effort to push it out. When you breath, extend your entire rib cage as much as possible.
3) After a few breaths, place your hand in front of your mouth and blow in it. Try to maintain the same air pressure for as long as possible. You will have to apply gentle pressure from your rib cage and diaphragm to keep the air pressure. Don't blow to hard, It's not necessary.
Do this a few time and be aware the muscles you are using to keep the air running.
4) Repeat the exercise using your mouthpiece this time. Hold long notes. You can try various pitches. You'll notice that it gets harder to stay relaxed as you get higher. Try to keep your torso muscles from getting tense as you go higher.
5) Now put your mouthpiece on your instrument and do the same thing with various notes. Once you feel comfortable, try scales and arpeggios.
Try to be aware that you're are using your entire body to produce your tone. Not only your chops. Feel which muscles are working.
You will have more power in the high register and more endurance if your lips are well supported by a good air stream that comes from the bottom.
You can improve your tone a lot by playing your music at a very slow tempo.
First, try to imagine the most beautiful sound in your head and then play the first note of your piece while still mentally producing that tone. After a while, your brain will send signals to the rest of your body that this is the tone it wants and your tone will adapt. Once you're happy with the sound, do the same with the next few notes. Remember to always focus on the tone you are making in your head.
It's a bit like doing tai chi on your instrument. You do everything in super slow motion. You don't need to play the whole piece like that. Just the first few passages. After a short while, your tone will match your mental projection quicker and you won't need to play so slow to get what you want. This will relax you and give you a nice smooth tone.
I am associate principal horn of the Singapore Symphony Orchestra and the author of the progressive methods. I'm happy to share my experience as a horn player and teacher with you.