You can do two things: - ensure you have proper posture - strengthen your "core"

First, do a web search for ergonomics and posture to make sure you're sitting in a neutral position. If you're not, you will be constantly using your back muscles (and neck, shoulders, etc) to maintain your posture when you sit, which will lead to pain.

Second, search for exercises to strengthen your "core". You should strive for simple exercises to maintain your muscles. Back pain is related to weak back and abdominal muscles.