Sitting on a hard wooden or metal chair—or even a soft, ergonomic model—can put a lot of stress on the lower back. That’s where a good seat cushion comes in. We’re not talking about the ones that fluff up the sofa, but rather models that are specially designed for comfort and proper posture. You’re probably better off with a contoured U-shaped seat cushion rather than a flat one, since it relieves pressure on the tailbone while contributing to good posture. In addition to shape, you’ll also need to choose between foam and gel filling, and the thickness of the cushion. Here’s how to choose the form that’s right for you.