Save yourself the cost of a posture-improving chair by purchasing an exercise ball? These inflatable balls require "active sitting" and may improve your posture by strengthening your core muscles.
But they may also cause you discomfort. You might not have enough space under your desk. There are no armrests. You may not be able to reach other items at your work station. And you may fall.
A better option is to choose a chair of good ergonomic design--with an adjustable back, seat and armrests. Wheels help too.
Make sure you vary your position, lean back, stand up and move around.