Endurance. Hmm…how hard can it be to write about that? Just practice a lot and you’ll have it. True, but it’s not quite that simple.
Endurance is affected by what kind of shape you’re in and how efficient you are in your playing. Of course, higher, louder, and longer all take more energy than middle range, medium volume, and music with lots of rests.
In a nutshell, play as relaxed as you can and use as little effort as possible so that the endurance you have goes as far as possible. As a plus, you also get your best sound this way and have your best flexibility. Use the air as much as possible to do the work — always take a full breath (unless the music is really soft), and breathe again before you get to the point where you’re squeezing the air out. This will help increase your efficiency.
On to the specifics…