Imagine stacking blocks on top of one another to build a tall column. In order to erect a tall stable column, you would have to stack the blocks on top of one another as closely aligned as possible. Now imagine that your body is made up of separate blocks made up of your feet, pelvis, chest, and head.
Start with your feet, sway your body forward and back and side to side until you feel that the block made up of your feet is resting in its most stable centered position. Now bring your attention to your pelvis and stack your pelvis on top of your feet. Now stack your chest on top of your pelvis. And finally, without changing the previous stacked blocks, stack your head on top of your chest. The result is an erect standing posture. Carry this feeling with you throughout your day to improve your postural awareness.