We all have to deal with this, in one form or another: GRAVITY! It pushes our shoulders forward when we're sitting, holding the baby, riding our bikes or running! It makes our head fall forward and walk in a less-than-head-held-high posture. That's if we're not doing anything to correct it, though!
First of all, we need to be aware of how we are standing, sitting, walking and so on. Then we can address our individual muscular strengths, weaknesses and imbalances in order to strengthen and stretch accordingly.
As far as sitting goes, try to do it less! If it's feasible, stand on a good surface with an ergonomically adjusted standing desk. Or, when you're sorting out your papers at home, squat! In addition, buy a stability ball and sit on that instead; it's a lot harder to slump at your desk if your core is actively engaged. Granted, if you work in a formal, conservative office, your boss may not like to see you sitting on what looks like an old-school children's toy (remember the hippity-hoppity?), but if you can do it, go for it! My favorite stability ball is the DuraBall, which you can order online at www.thechekinstitute.com. It's rugged and durable. Just refer to their size chart and see which ball fits with your height.
If you are in a situation where you have to sit in a chair at a desk, at the very least, try to stand up and stretch with some regularity to keep the blood flowing and the muscles from getting tight. Lean your forearm against a wall, elbow at shoulder height, and turn away in the other direction for a good chest stretch. Interlace your fingers behind your back and roll forward at the waist for a great shoulder stretch. Apply gentle pressure on your neck using your arm for a lateral neck stretch. Roll your ankles front to back and side to side. Go for a quick lap around the hall, again, just to keep the circulation going!
Keep as active as you can, as often as you can, and don't let gravity weigh you down!