I've been playing the cornet for about a year now. I have found that I can play for about five minutes and then I can't play anymore. When I come back five minutes later, I can play just fine and I play for half an hour to an hour. I usually just play Stars and Stripes Forever to warm up (the Solo Bb Cornet part, about half way through). I hit the high Bb in the last strain and crawl a few more bars and I'm wiped out. When I come back five minutes later, I can play it as much as I want and I'm fine.
I've also noticed, when I try higher pieces for warm-up, such as the Stars and Stripes Forever as I mentioned, I have trouble with the low notes for the remainder of the practice. However, I'm fine with the high notes. If I try a piece with a lower range, e.g The Thunderer, I can't hit the high notes for the better part of my practice, but I can hit the low notes just fine.
I was wondering if anyone knew of a good warm-up technique. Currently, the high Bb flat is my limit as far as range on a good day.
EDIT: What other things should I work on during it? I'm looking for specifics. What scales should I work on? How far should I press my range? Should I take breaks during it? etc...