We often unwittingly make little adjustments to our posture as we start to play a note. I always preach,
Stand tall, bring the instrument to you.
In spite of this, those "bad posture" habits can be very strong! Sometimes I ask a student to simply go through the motions of bringing the instrument up to play while watching themselves in the mirror.
The instrument just goes up and down...up and down...while the body stays in a nice, well-balanced position.
This may seem silly, but it can be quite remarkable when, after repeating this several times, the student actually goes to play a note or two. They often observe that the notes feel easier and sound better.
This repeated up and down motion reminds of those little gizmos that simulate a bird dipping down to take a sip from a bird bath. Personally, I always called them bird bobbers but apparently the rest of the world (and Wikipedia) calls them "drinking birds."