The back should be upright and relaxed, not curving over but not rigid either; imagine the shoulder blades drawing back and down while the spine lengthens and the head draws upwards. The arms are relaxed from the shoulders, hanging straight down with the hands placed on the desk, keyboard or mouse so that there is a 90 degree angle at the elbow. The head is not poking or peering forward, it is sitting relaxed on your neck which is also relaxed and upright. Think about the jaw, it should be relaxed with the tongue resting on the floor of the mouth.
When sitting in a chair, the feet are placed on the floor with the knees at a 90 degree angle while the back is upright. In order to create this a footstool can be used under the feet. Make sure sitting bones at the base of the buttocks are in contact with the chair you are sitting on. The surface should be firm enough to support you without creating discomfort. Various ergonomic chairs can assist you in creating a supported posture. This posture only requires our attention every now and then to ensure the body is aligned. In this way we can focus on our work and not distracted by the body. If our posture is aligned as indicated above, at the end of the day we will feel less aches and pains and we are less likely to suffer from back & neck problems as well as RSI.