I see guys around me who aren't really bad players, but are a pain to sit next to because of some of the douchey things they do in rehearsal and while they're playing. Mostly just bad habits, these things can really improve the dynamic of the ensemble if you just think a little.
1. Don't play while your slide is moving! (unless it's some glissando effect written in the music) Yeah, that seems obvious but some players don't realize it. Lazy, glissy playing not only sounds bad, but it's out of tune too. One guy I see just doesn't land on the note - he eases his slide into the position and I don't think he realizes he's doing it and his intonation is terrible because of it. Maybe we should all video ourselves playing something simple and watch what's happening since we all have a cell phone with a camera. It'd be pretty easy to find out for sure.
2. Don't play so damn loud ALL THE TIME! The trombone is a loud instrument and has been getting louder due to the trend of building horns with bigger and bigger bores. A lot of players tend to just play loud all the time because they can and think everyone wants to hear how great they are or some other douchey reason, but if you play with some restraint, you not only save your chops, you're that much more dramatic when your awesome volume is called for.
3. Pay attention in rehearsal! Yeah, that means shut your pie hole and put your phone down. An old guy looking at Facebook and texting during rehearsal like a kid is pretty douchey, especially when he doesn't know where we are in the music because he wasn't paying attention. All the talking is the same - if you were running the rehearsal I'm sure you'd think it wasn't so harmless.
There are other things which I might add as I think of them, but these are the biggest things I consistently see around me in sections in which I play. I wouldn't mind if any dear readers of this would add things in comments if so inclined.
