Good posture helps you breathe better and play with less tension.
Distribute your weight evenly on both feet. Avoid leaning to one side.
Your head should be balanced on top of your spine, not tilted down or to the side. Bring the flute up to you — do not bring your head down to the flute.
Let your shoulders drop naturally. If they creep up toward your ears while playing, pause and shake them out.
Your elbows should be slightly away from your body, not pinched against your sides or sticking out too far.
Every few minutes during practice, do a quick body scan. Are you tense anywhere? Relax that area and continue.