Many office chairs have a pneumatic cylinder that enables the user to adjust the height of the chair. Occasionally the cylinder may leak, causing the pneumatics to fail and allowing your chair to sink slowly as the day progresses. This can be incredibly frustrating, but is quite easy to fix -- and you won't need to go back to the manufacturer for expensive replacement parts.
Sit in the chair and maneuver the chair until it is at your required height. Measure the distance from the base of the seat to the floor, marking the position of the cylinder.
Drill a hole in either side of the cylinder shaft at this marked point. Insert a screw in either side, holding the cylinder in place permanently. Ensure that you use long screws to provide maximum support.
If you don't have a drill handy, cut or break some wood and lock the chair in a raised position by attaching the wood to the side of the cylinder with duct tape. It should fit snugly between the underside of the seat and the wheels' base.
Use a lubricant if you need to take the chair apart.