OK, so now you know basically how you should position your body when standing and seated. But how can you remember to maintain good posture after years of living life slightly slumped?
First, establish what proper posture feels like.
For good standing posture, position your heels against a wall, then stand up straight and let your shoulders and the back of your head gently make contact with it. There, that's standing up straight.
For a seated posture baseline, put your butt against the back of the chair, touch your shoulders to the chair's back, level your chin with the ground, get those legs bent to 90 degrees, feet flat, and then make sure you can slide a rolled magazine or small towel behind your back. Good.
Next, make a posture mantra.
Come up with a simple phrase you think to yourself every time you stand up or sit down. Something like: "Standing up, straighten up" or "Sitting down, feet on the ground."
If remembering to have good posture isn't working, don't worry, there are gadgets and gizmos that can help you.