To help your back, follow these tips for good posture when you're sitting:
Always maintain the three natural curves of your back.
Sit back in your chair so your back is supported.
Feet should be supported on the floor or footrest, not dangling or resting on the base of your chair.
Legs should not be crossed.
Avoid slouching, slumping, bending forward or backward, or twisting at the waist.
Do not sit for long periods; stand up and stretch.