Untill you change the seat, the rest are all just compromises..
1) CG lock can be helpful
2) Bracing your left leg on the dead pedal when not shifting is helpful
I used to latch my 3pt, then pull up on the shoulder part of the belt and found that was sufficient. I never ran a CG lock
However, if you hop in a car that has, say GT3 seats, Recaro or Sparco race seats, you will quickly realize just how much of your energy is going into "staying still" instead of driving.
you must have really big cheeks to leave marks on the window
(oh yeah, no matter how cold, you are always faster with windows and top down
