1) Sit with a neutral pelvis. Sit evenly on the right and left side of your pelvis and evenly touch the right and left back side of your pelvis to the car seat. Imagine the pelvis is a bowl of water and the water doesnt spill forward or back (not too much arch like a gymnast and the tush shouldn't be tucked under flattening the lower back curve.)
2) Sit tall, & adjust your rear view mirror (and side view mirror) so you have to have correct posture when you drive.
3) Sit tall & Stick/Pin the shoulder blades & back of head to the car seat. Imagine velcro on the shoulder blades and back of head so they touch the back of the car seat when you drive.