Choose a chair to match the type of work you do while at your desk. Classify your chair and desk usage, and purchase a chair fitted for that kind of work. For example, if you perform light, clerical duties and are up and down frequently, with no need to be mobile while seated, you do not need an executive or heavy use chair.
Select a chair to fit your frame. Make sure you have plenty of sitting room in the seat of the chair.
Test the chair. Sit down in it with your hips and back against the back of the chair. Assume a straight sitting work position with your feet hip-width apart and flat on the floor. Raise and lower the chair until the seat level matches the length of your lower legs, and your feet rest flat and comfortably on the floor.
Place both of your arms on the arm rests. Take note of how the chair back and seat fit the contours of your body. Notice where the front edge of the seat meets the backs of your legs, and whether or not it is comfortable. Make sure the front edge of the chair does not restrict blood flow at the bend of your knees to your lower legs.
Do not buy the chair if you feel discomfort in your lower legs and feet even after you have it adjusted to the proper height. This may point to a front edge of the seat that is too hard and too elevated. Avoid these chairs since this design flaw hampers sufficient blood flow to your lower legs and feet.
Notice whether or not the back of the chair fits your back properly or not. Feel for any uncomfortable bulges in the lower back area. Make sure the middle and top of your back meet and fit the back of the chair when you sit straight.
Note whether the arm rests meet your elbows comfortably when resting them. Are they too high or low? Sit for a couple of minutes, just to get the feel of the chair. Go through the motions of what you normally do in the chair while at the desk. Check the overall comfort level.