Is it while sitting or standing?
I'm 6 feet tall myself (Well, 5 feet 11 7/8 inches if you want to nitpick) and I've been living with upper back pains since the beginning of the year because of that slouching issue.
Posture, either sitting or standing is always a manner of habit, and breaking that habit requires a lot of effort. There's no 'easy trick' around it, I'm afraid.
For standing up, it's best to maintain a posture that positions equal weight on both legs, if you're looking in the mirror and you naturally hang on one feet more than the other, it's bad. Normally, your back should form a slight 'S' (Check a few pictures of it on the internet for reference), get used to bending your legs more than your back to reach things.
For sitting up, it's a little easier, you need to adjust everything at the right level, so your legs bend for a 90~110 degrees angle at the knee with your feet resting on the floor.
If you work on a computer (Like me!), your chair should have a back rest that helps maintain the natural curve of the back while supporting most of it --- in my case I found it's right around the position of my elbows that it needs to be. Keyboard and mouse should be at the same level as your armrests, so you don't bend over constantly to type or use the mouse.
Sitting straigth on your chair, your eyes should be looking around the top quarter of the screen, if you need to look at the bottom of the screen, bend your chin inward so the head moves, don't slouch so your head gets level with the text.
Avoid resting your head on your arm, it weakens your back and shoulder muscles since it's often just your stiff arm bone that supports everything when you do it.
Also, stretching your back muscles daily helps add some elasticity and a modicum of strength to them, helping them support you. There's plenty of exercises to befound online for this (No need for it to be a training regiment, for those on a tight schedule)
All a shame I started paying attention to all this when I started having issues.
Last edited by Kildread : 09-06-2010 at 08:30 PM.