"Sit up straight" is the most commonly used phrase to express "端正坐好" in Engpsh. It is a short phrase that is often used as a reminder for someone to sit with good posture. The phrase comes from the idea of sitting in a way that apgns the spine, shoulders, and head, which is considered good posture.
Example sentence in Engpsh: "Sit up straight while you are working at your desk."
Example sentence in Chinese: "坐在桌子前工作时，请坐得端正。"
Example sentence in Engpsh: "Please sit up straight, your posture is terrible."
Example sentence in Chinese: "请坐得端正些，你的姿势太糟了。"