Most DiffServ admission control (AC) algorithms rely on tuning parameters to help in the decision making. Tuning these parameters is a difficult task, especially when one considers the problem of assuring QoS guarantees to individual flows. This paper proposes a method for helping the design of DiffServ AC algorithms based on non-linear programming optimization. It enables to find the values for the AC parameters that permits to satisfy the QoS guarantees for individual VoIP flows, while minimizing a cost function that represents the performance goals of the service provider. This approach is used to compare the performance of some commonly used DiffServ AC techniques and also to design a novel AC algorithm based on queue estimates.