x2 - especially the part about standing and sitting.
The best posture is the one that's frequently changing. No matter how ergonomically-correct your chair, keyboard, mouse, etc. are, the simple fact is that it's bad for our bodies to sit down (or stand in place) all day. Try for a mix of standing/sitting/walking/moving around during your work day. According to Bill Hartman, physical therapist and co-owner of I-FAST gym with Mike Robertson, tissue creep (negative postural adaptations) sets in after just 20 minutes of being still. So try to get up, stretch, and move around at least 3 times an hour.