An excellent book for the comeback player is Claude Gordon's Physical Approach to Elementary Brass Playing published by Carl Fischer. Some general guidlines:
1. Do not be in a hurry. You are building. Lay down your foundation carefully and patiently. Let the cement dry before moving on!
2. Focus on breathing first. Do simple breathing exercises everyday. There are good ones in the book mentioned above.
3. Next is your tongue. Use the "K" tongue modified technique of tonguing. Leave the very tip of your tongue at the floor of your mouth just behind your lower front teeth and use the part of your tongue between the very center and the very tip to release the attack from behind the upper front teeth. This is discussed by Herbert L. Clarke in his Characteristic studies text (pub.-C.Fischer-02281) on page 5, paragraph 7. With time and proper practice, this technique will make your tonguing, along with everything else, more efficient.
4. Practice everyday at about the same time. Use a routine. Routine=the same collection of exercises and/or songs/etudes for a set period of time. Time=1 week or 2 weeks at a time before changing the routine.
5. At first do not try to play to softly or loudly. Focus instead on "moving air through the horn" i.e., the comfortable feeling of blowing without holding back.
6. Don't get hung up too much on buzzing with or without the mouthpiece. Too much analysis causes paralysis.
7. Stay on track. Make up your mind in the beginning which books to use and stick with your plan. Don't change unless after reasonable time you think you may have a problem or your not getting results.
8. Practice. Don't worry. You cannot progress faster than the natural processes will allow.
9. Do not practice only one kind of exercise. Mix it up. Long tones (not too much), flexibility, single tonguing, etc. Do not rush range in either direction - pedals or ascending notes. In time, with proper practice, they will come. "If you build it (properly) they will come." Just couldn't resist that one <G>!
10. I recommend my book, Fundamental Flexibility Studies.
Check it out at http://expage.com/page/fundflex
Author of Fundamental Flexibility Studies
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