Add a seat cushion
Use a lumbar support pillow
Adjust the chair height
Adjust the backrest angle
Add armrest pads
Improve seat depth with a cushion
Use a footrest
Replace hard casters with softer ones
Tighten loose parts
Add a headrest
Use a breathable seat cover
Reupholster the seat with softer foam
Add a gel pad
Position the chair correctly at the desk
Take regular posture breaks