Is it really possible to change your posture?

I've always wondered how if you got someone with bad posture to temporarily hold good posture, would an expert me able to tell if they're faking it or not? We're always told to hold good posture, as if to imply that some day we'll no longer need to think to hold it! But that day never comes. A physiotherapist recently said to me that one can't really change their posture.

I look around me at college, and I see young people hunched into unhealthy positions with their laptops inches from their faces, sitting down all day.

Another thing, is the question whether tall people are more susceptible to forward head posture? For me (as a tall man), I find that if I have to bend down a bit for something, and then walk over somewhere else to bend down for something else, that I'll actually sort of stay hunched while walking from one to the other! Interesting, right? That, right there, is the problem... that so many of our tasks involve us being slouched that we just sort of stay in the slouched position in anticipation of such tasks. Like say, cooking the dinner for example. Most counters tops are too low for people over 6 feet.

Here are some possible habits that might contribute to this trait:

Cycling - handlebars too low
Desk position - back rest too far back, so therefore the head has to tilt forward to remain upright
Preparing food on counter-tops
Mobile phones - looking down at them
Seating postion when driving
Playing nintendo games