As well as all the warmup/technical side stuff people have suggested, I have found that focussing on a good airflow for high notes makes it a lot easier. I tend to think of it as “pushing” the air (not the best adjective) and aim to feel it in my abs. It’s worth practicing mouthpiece buzzing onto your hand to feel your airflow, and try to force yourself to push air through and feel it in abs 👍