Common problem: we adjust our bodies to the instrument. Whether standing or sitting, keep your head balanced above your center of gravity. Bring the instrument to you.
Do the Dippin’ Bird
That little bird bobs up and down but the legs stay in place. Watch this video: I keep my posture balanced over my center of gravity while the trombone moves up and down. I don’t adjust my posture to the instrument.
This can be a very useful exercise.