For me, when I get tired on a gig, my lips get fatigued and my chops spread. The aperture gets too large to contain the air column and then I resort to mouthpiece pressure to push the aperture closed. We all know what happens next. Crappy, splatty sound, loss of range and flexibility, and a quick decline in endurance. Some lip flapping can provide temporary relief, but for when I’m done, there’s not much more left. My best playing of the day is done. To counteract this from happening it helps me to actually think about my aperture from time to time when playing. I remind myself periodically to keep my lips closed (not forcefully but gently) when I start a note, letting the air create the necessary size of the aperture. When I do this, my sound is much better and consistent and I generally have more endurance. Keeping control of one’s aperture is a key component of finding the overall balance/coordination necessary to play as effortlessly as possible, which for most trumpet players is the key goal so that we can actually focus on making music.
So with this concept in mind, check out this cool little aperture “test” I found online (can’t remember where, sorry). The idea is simple, as is the actual music (don't forget to use a metronome). I like to play through this exercise a few times at the end of a practice session. If I had been playing correctly and efficiently I can play this little tune very quietly, with a soft tongue, and a nice pretty tone throughout. I can’t play the whole thing twice like the note about Herbert Clarke says, but I can consistently get 24 or 25 bars before I run out of air. And it feels good. Playing through this exercise during one’s warm-up could also be beneficial; making sure that one’s aperture is only as open as necessary and that the air column is moving steadily and consistently. I know it sounds simple when playing through this, but the quiet, focused playing really allows the body to do what it needs to do to find the proper balance/coordination. If you practice this, please let me know what you think. It’s not the Grail or anything, but I do think it’s quite helpful.