The paper proposes design criteria for the choice of the amount of prestressing. Based on an extension of the well known load balancing method, the proposed criteria permit a simple and effective design of the amount of prestressing at serviceability. The paper also discusses the favourable effect of compressive stresses induced by prestressing on cracking and water-tightness of PC structures. In situ measurements as well as extensive test results demonstrate that the proposed criteria lead to structures that are more durable and less prone to an increase in deflections and cracking over time.