Ride from your elbows. Then you won't tense up your forearms
My instructor used to put tape on my thumbs, basically had to be able to see the tape and if I couldn't I knew to correct myself and eventually it just becomes habit
Train by having a whip between my thumbs if you get what I mean? You hold the thumbs on top, then lay the whip under one thumb and the other one. It's pretty hard but when you've done it a few times it should've helped!