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.
Many musicians, actors, dancers and athletes take Alexander technique lessons to improve the way they use their body. I have personally done it myself for about a year in Switzerland back in 2001-02. AT is not only about having the right posture when you play but also about making the right move with your entire body to achieve an optimal result. It teaches you not to focus on the actual results but on the means to get there. If you do the right move, you'll eventually get the right result with time and patience.
We very often take shortcuts to reach notes or play a difficult passage. Although you might actually get what you want in the short term, you can easily develop bad habits from forcing things which will hurt you in the long run. AT can help you unlearn some of these bad habits and replace them with good ones.
I was always amazed how good my playing felt after each Alexander technique lesson. Some conservatories and music faculties offer it on their premises. If you are lucky enough to be in one of these schools, take advantage of this. If your school doesn't have an AT teacher, make a little research to find one in your area. You won"t regret it. You'll immediately feel a difference in your tone and it'll greatly improve the fluidity of your playing.
Here is a video of an Alexander technique masterclass to give you an idea of what it can do for you. You can find many more about it online.
Here's a little experiment: make a fist with both hands and squeeze as hard as possible. While doing that, try to blow air. How does it feel? Not very good of course.
Now relax both hand and blow. See how natural and comfortable that feels.
We tend to press our valves or pistons too hard when we play. Especially for high or difficult passages. This creates tension in the harm which affects the rest of the body and stiffens the air flow causing problems in the high register.
Take a couple of minutes to practice pressing your valves gently with just the minimum amount of pressure. Trombone players can practice holding the slide gently and moving the harm fluidly. Then play on your instrument this way. Your playing should immediately feel more relaxed and comfortable. You might miss a bit more notes at first but it'll only take a few minutes for your embouchure to get used to this new relaxed way of playing.
This will help your tone in the high register and improve your endurance.
Don't we all love these high and soft passages? They make us look so good all the time! Just kidding of course!
I'm afraid there is no trick that will guaranty a 100% success rate but there are a few things that can help you.
First, you need to plan your dynamics according to your artistic judgement rather than your physical abilities. If you just try to play as soft as you can, you'll always try to get softer and softer and your playing will become uncomfortable. This will cause you to miss notes and create more stress. You should always play as soft as you SHOULD instead. This way, you will get to a nice soft sound and keep it there unless the conductor asks for less. They don't always do by the way contrary to what most people say! By having a "bottom" to your dynamics, you give yourself a realistic goal and take some stress off your shoulders.
Second, take a good breath before playing. When we see piano written in our part, we very often have the reflex to take a small breath. It's much easier to play a soft passage with full lungs.
Third, let go a nice piano sound rather then restraining your forte.
These tricks can help you playing soft in the high range but there is no guaranty. You need to be patient with this aspect of your playing. Avoid over-practicing a passage before a concert. You'll eventually miss a note if you try to play it 100 times in a row which will make you more nervous. Concentrate on what you have to do to make it work and trust yourself.
I had a few students who just couldn't help breathing through their nose. It just didn't help if I kept telling them to breath through the mouth so I had to try something else:
I tell them to stop playing completely when they need to breath. Then they take a deep breath through the mouth and continue. They will take the habit of breathing through the mouth after a week or two and won't need to think about it anymore. Of course you can't play like this in rehearsals or concerts but you can do it when you play by yourself. Using that method, the few students I had with this problem learned to breath through their mouth in about two weeks without me having to remind them every minute!
It is important to have a relaxed and fluid air flow while you play. I had the chance to have a few lessons with Mr. Vincent Chicowitz (former principal trumpet of the Chicago symphony) who taught me to practice just the air movement without any instrument or mouthpiece. Basically, you just practice your music blowing the rhythm as you would blow in your instrument if you were playing normally. Groups of slurred notes are blown like one long note like a tie over. You practice a difficult passage like this two or three times and then you go back to your instrument. It feels great! It really helps for passages in the high register. I personally do it every time I feel my air is a bit stiff.
I personally practice on my mouthpiece alone every day for a few minutes as part of my warm up routine. I do one or two octaves glissandos to increase flexibility and fluidity. It is also very useful to play a difficult passage on the mouthpiece and then play it on the horn. It allows you to focus on the actual movement of your lips and air flow. It's also a good way to find out if you hear the music correctly.
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.