SIT UP STRAIGHT
Flatten your lower back into the chair to ensure that you're neither overarching nor rounding at your waist. Pull your shoulders back (imagine your shoulder blades moving toward each other in the midline of your back). Position your seat so that whatever it is you need to see is directly in front o1 your eyes. The idea is not to have to tilt your head up or down for an extended period of time. Be especially careful at the computer or when relaxing on a couch or armchair.
STAND UP STRAIGHT
But don't lock your knees when you stand. Bend your knees very slightly to prevent your hips from rolling forward. This takes the pressure off your lower back. Imagine there's a string attached to your head that's pulling you upward, as if you were a puppet. Pull your shoulder blades back to square them off so that you could run a straightedge from the back of one shoulder to the other.
When walking or running, hold your head high and look toward the horizon, not down. Don't cross your arms in front of you, as that makes your muscles work harder to recenter your torso with every stride.
When resting, lying in the fetal position on a firm but accommodating mattress is best for your back. Don't tuck a pillow behind your neck, because that can put a crimp in it. Be careful reading or watching TV in bed, as both tend to put the neck in an awkward position (especially with pillows propped behind the head) for extended periods of time.