You can make use of a balance ball as a sitting position during the third trimester of preganancy, as this can be of immense benefit to you. But make sure to buy one that fits your height. It can help prepare your pelvis for labor and even help position your baby properly.
In the workplace, while sitting at your desk, adjust the height of the chair to level with the table. Sit near your desk so you can rest your arms and elbows on the table or chair. This will help relax your shoulders.