Of late, I spend a few minutes when I first sit down doing what you suggest, letting attention go to the area of my sacrum and its movement vis-a-vis the ilia. I also rock back and forward, sideways, and observe the rotation that naturally occurs. Then I think of the basic Alexander Technique instruction, “Let my neck be free.” etc. Once I’ve adjusted and settled in this way, I put my hands in the mudra, and start my zazen. I also use double mats below the cushion at home, or even bed pillows.