Pick up the horn. Play, warm up, noodle around, get comfortable. Nothing wrong whatsoever with a wise, careful beginning to sessions. But now with that accomplished, it's time to sound like a first class player.
What's up? Charlier, Irons, Caffarelli, Mahler work, Neruda, or even scales and arpeggios. Whatever is on the stand, it doesn't matter. The days of junior high playing are history, so get over it. Filter all the junk out beginning with your first note. Listen similarly for all notes in the phrase because it's all supposed to be good.
Now here's the key: As valuable as a no-junk mindset is, you can't survive unless you add to it a monster musician. Otherwise you'll die of boredom, or your listeners will. The monster will dictate the quality of playing, and will quickly take care of the junk. We work feverishly on eliminating the junk, and still fall short because the hero of the day never gets to the stage. The hero inside must first be tamed, unleashed, and allowed to rule. This includes practice sessions, rehearsals, concerts, and valuable time spent away from the horn.
Remember the first time you heard Mahler 5? The next thing was a bee line to the practice room to imitate what you heard. The monster performer trumped the garbage man. Garbage is quickly shunned when the monster rages.
So, what are you thinking about, making no mistakes, playing perfectly clean, or about how wonderfully you are about to play? You decide. The horn will respond accordingly. Great music starts long before it is ever heard.