My son has been working on not letting the lift leg drift toward 3rd. When the leg drifts the upper body follows, he gets tilted and you get the usual assortment of problems that come from being out of balance. A suggestion he got from his instructor recently was to keep his weight on his heal, rather than the ball of his foot on his post leg.
After quite some bit of time trying to work through how to fix this balance problem, this simple adjustment has worked wonders. The best part is you see it show up in his command of the strike zone with one more variable in the motion more fully under control. He got through two innings on 13 pitches his first time out after the adjustment. Awfully small sample size, I know, but he's also feeling alot better in the pen and has confidence he can hit the mit where it's placed now.
Eight months ago he was a mechanical mess and they've been cleaning up bits and pieces along the way. It was nice to see this lingering problem get cleaned up with such a simple solution.