Sit all the way back in your seat, and feel your sits bones root down in the chair.
Sit back all the way so the backs of the shoulders and spine are supported by the back of the chair.
Sit with both feet flat on the floor - we tend to cross one leg over the other, and it's usually the same one (skewing the hips). Feel both feet firmly and evenly on the ground.
Turn your computer or work to face you, instead of craning your head and neck to face your work.
Sit high enough so that the shoulders are above the elbows, and the elbows above the wrists.
Roll the shoulders back and down multiple times per day, so they stay low out of the ears, and there is still space across the chest.
Draw the low belly up and in to support the low spine whenever you remember.
Take lots of breaks! Stand, stretch, walk around.
Breathe!!! We so often hold our breath, especially when stressed or concentrating. You might even take your hands to your low belly so you can feel your breath start in this place (rather than the chest, where we have shorter, more shallow breaths).