Alternate sitting and standing. The key is to move and not stick to one posture. I keep a bottle of water handy. If you drink water regularly it forces you to take bathroom breaks, forcing you to move.

Try to distribute your weight evenly to both legs. Don't lean on one leg too much. Try to engage core muscles.

Stand on a mat that has some cushioning.

This is subjective. Eye direction looking straight matches top of the monitor.

Elbow at right angle to keyboard mouse