Obviously we need to keep good posture in mind, but the one single change that I made helping me play violin for countless hours without getting tired is to learn to sit on the edge of my chair. Having worked in professional full time orchestras for 25 years I have been playing while sitting for countless hours to say the least. I used to sit back in my chair thinking that I could relax but was always tired and couldn’t wait for the next break. After making a very conscious effort to sit on the edge of my chair with my back arched, I rarely feel tired and time actually flies during rehearsals like it never did before. It takes more initial effort to sit correctly but makes playing much more pleasant and precise.
If you don’t already, DO IT.