Habitability and robustness have been noted as important qualities of natural-language interfaces. In this paper we discuss how these requirements can be met, in particular as regards the system's ability to support a coherent and smooth dialogue. The discussion is based on current work on customizing a dialogue system for three different applications.   We adopt a sublanguage approach to the problem and propose a method for customization combining bottom-up use of empirical data with a global pragmatic analysis of a given application. Finally, we suggest three design principles that have emerged from our work called the sublanguage principle, the asymmetry principle and the quantity principle.