remind yourself hands up during a turn so you can maintain your rhythm
Hold a crop under your thumbs (so that it is laying across the neck and you're holding it with both hands)! This helps me not get too much on one rein and will hopefully also help you keep them centered
I have the same problem. My instructor tells me to imagine I'm carrying a tray and if I drop one, the tray will fall.
I had the same problem... My teacher said me to think about keeping it fixed and remembering all the time about the hands and now I have solved that problem! I hope this advice will work!