This paper particularly addresses the difficulties arising from formulating models for dynamical problems in such a way that they can be treated by appropriate solvers. One of these difficulties is that different types of solvers are recommendable for different questions within the same situation. Therefore it is recommended to use solver-independent specification methods. Since specification and respecification can be time-consuming and boring, it is also recommended to develop rule-based specification tools. The paper is illustrated with a sketch of possible specification methods for manpower policy making and for goods flow control.