First, make sure you’re sitting back in your chair with your feet flat on the floor. Imagine that your coccyx (tailbone) is made of lead and pulling straight down into the chair. Make sure your spine is in neutral and you can feel the middle of your back lightly pressed against the seat back. Your shoulders should be relaxed but not slumped. To get a feel for that position try some leg lifts. Lift each knee alternately up, placing it smoothly down again with the foot flat on the floor. You should be totally stable, if you can feel your coccyx moving, then check that you are sitting back and in the neutral position—you may be sitting too far forward with your pelvis tilted. If you’re not sure about neutral when sitting, then try lifting the knee right up towards your chest. As you do you’ll reach a point where you can feel the pelvis tilt, and the coccyx slides towards the front of the seat. Hopefully that feel for being out of neutral should help you settle into neutral.
With your pelvis and back sorted out, the next issue is your shoulders and the neck strain that all too easily results from tension and poor posture. Hunching your shoulders upwards is a shortcut to tension and trouble as it tightens the trapezius muscle at the top of your back and that transmits its strain into the back of the neck. If you feel tense (and who doesn’t at some point in the working day) then try this.
Slide your shoulder blades towards each other and then down and at the same time extend your neck. You should feel an immediate easing of the pressure on your spinal column, shoulders and neck as well as feeling as if you’ve just grown an inch. Next time you want to shout at someone, try doing that first. A really great time to try the sitting exercises is on the bus/metro or at the traffic lights on the way home. Because we’re tired we’re likely to slump and take all that tension back home with us. Try and make it part of your daily routine to ease that pressure out as a way of leaving work behind before you get home.
Here's another idea for you...
‘The lift’ is another exercise you can do at your office chair or on the bus. The aim is to work on zipping up from the bottom of the pelvis towards your ribs. While sitting upright, imagine that there is a lift on your pelvic floor. As you breathe out try to take that lift ‘up’ to the next floor—you should feel your pelvic muscles go taut. As you take the ‘lift’ further up the floors from ‘first’ to ‘second’, you should feel your lower abdominals tighten. Higher than that and you risk the six-pack muscling in on the action.