Sedentary profession in an office

Modern times bring also technical progress and great deal of relief from manual work, however there is also much higher demand for workers behind computers. These people do not perform any hard physical work, but on the other hand they greatly burden their body. Especially by staying in one position most of their day, and most have also adopted wrong habits, which are harmful to our health. So how should we sit correctly, in order to not hurt our body?