Having had a 6'4" friend who had slouched his whole life (as many kids who grow tall fast do) and a girlfriend (now wife) who had a pretty extreme pelvic tilt, these things CAN be corrected.
The first step is to stand up straight. Seriously. Nothing else you do will work if you don't stand up straight, ALL THE TIME. Constantly re-adjust your posture, look at yourself in the mirror and PRACTICE standing up straight. Get a feel for what it is like when you are standing up straight, and pay attention to make sure you are doing that. If it's possible, get a partner/friend to remind you to do so (I actually got my girlfriend's work mates to tell her she wasn't when her posture slipped).
Standing up straight is going to be both difficult and uncomfortable. It's going to be uncomfortable because it's no longer your 'natural' way of standing, and hard because it's going to take constant (and I mean CONSTANT, rest of your life every waking moment) effort.
For the comfort part, do it more. The more you stand up straight, the better off you are and the easier it gets. Don't 'slump' in exhaustion or to relax. What will help you here is stretching to correct your imbalances. The muscles in the direction you are slouching forward are going to be tight and pulling your posture back to your current 'normal', so stretching them, lengthening them, relaxing them, ect, will help. Spend some time laying flat on your back as well, and try pushing your shoulders back to the ground (this is what you should be doing instead of slouching into a chair to relax). Also, sleep with a much flatter pillow.
For the hard part, lift. It's going to take constant muscular effort to pull yourself upright and maintain that, so start lifting and focus on an upright posture and strengthening your back (lower and upper). People often recommend lifting to fix posture issues, and this is the reason for it. Again though, this will just give you the strength and stamina to stand up straight, YOU need to actually STAND UP STRAIGHT all the time to make it work.
For another big tip, if you sit down a lot, figure out a way to do it right. Standing up straight for 30minutes a day isn't going to matter if you're slouched over a keyboard for 6 hours. Move the Keyboard to the edge of your desk. Get your elbows off the desk so they are at your sides, pull your shoulders back. Since you're tall, try elevating your monitor (ie avoid laptops). If you get sore from doing this, your back hurts, ect, it's because you are fighting against your current 'bad' posture. It will become easier as you get stronger (from lifting), your muscle imbalances start to go away (from stretching), and it becomes habit.
Posture is something that can be fixed, but like everything else, it takes real work.
but it took me months and months of telling myself, "don't rest your face in your hands" "dont touch your face" apparently you touch your face 3000 times in a day! that's insane! i bet i only do it 1000 times. 
