I used to put a whip across the top of my hands (hold it with your thumb). It forces you keep your hands up, coordinated and straight.
I used to be really bad with this but my instructor taught me by first practicing with something small like a pen or whatevers around and you put it under your arms so they stay by your side and you have a bend in your elbow and usually (with me) my hands would raise when my elbows were bent this also helps if you have "chicken wings".
I like to think of keeping my hands at the same height as my belly button. Also helps to keep a consistent bend in the elbows 😁