I play tenor, and I've always had trouble with tone and intonation on high notes (around middle A and up). For a long time I used to bite down on these notes, even though I knew it was a bad habit. I've weaned myself off the biting habit recently and I understand how to get air support from the diaphragm and chest, but I can't sustain it for very long. Especially playing long phrases that end on a high note, by the end of the phrase I often run out of air. Part of the problem is that I have a mild case of pectus excavatum (chest slightly sunken in), which means I can't fully expand my lungs. I practice long tones and overtones regularly, and they definitely help. What are some other exercises I can try?