In this paper we describe a method for the derivation of the constitutive behavior of rate-dependent materials from two potential functions. Once the potentials have been specified an entirely standardized procedure can be adopted to derive the response, and no additional assumptions are necessary. There is some freedom in the choice of which potential functions to specify: the first is one of four energy potentials, and the second, which specifies the dissipative behavior, is either the force or flow potential. The method allows alternative models to be quickly compared and set within an appropriate hierarchy. This work extends previous work on rate-independent materials, for which the force potentials take a special form. We illustrate the method with four examples of different viscoplastic models.