I went to a chiropodist because my posture is bad and he gave me these tips which I think do help.
When you walk make sure your hands are rotated so that the sides with your thumbs is facing forward. This rotates your shoulders back slightly. If you walk with your knuckles facing forwards your shoulders are rotated downwards.
Every now and then during the day stretch up and move your arms as if you are reaching up for the rungs on an invisible ladder. Climb 20 or 30 'rungs'.
Stand in a corner. Place your hands at eye level on the walls and press outwards while leaning forwards. Repeat at shoulder level and slightly lower.
Stand or sit up straight. Place your hand on your abdomen where your diaphragm is. Breathe making sure you can feel your diaphragm move.
Get an exercise roll. Kneel in front of it and clasp your hands as if praying. Rest your forearms on the roll and roll it backwards and forwards.
Make a sort of crab shape where you are balanced on your feet with your hands reaching behind you onto the roll. Lift yourself up and down a few times.
From Callenetics (showing my age here), rotate your arms up in front of you until vertical with your ears, then stretch out to the sides and down. Repeat about 20 times. Reach out behind you with your arms and try to touch your thumbs together, you won't, but it forces your shoulders back.