Imagine a force simultaneously pulling your tailbone straight down, and the top of your head straight up, and tense your middle to maintain that elongation.

Yes, don't try to straighten your back with voluntary muscular contractions, find something to visualize or focus on that straightens out your posture.

I've found that focusing on particular points on my body tends to straighten out my posture. The best point I've found is that little notch in the middle of your collar bone at the base of your neck. Tends to relax the sternomastoids, putting the head in proper alignment with the spine, and the spine entire in proper alignment.

Likely different points have different effectiveness for different people, given what's wrong with their posture in the first place.

Other points to try - ear holes, top of the head, sternum, sacrum. Try shifting from one to the other, and feel how your body adjusts.