Recent work demonstrates the potential for extracting patterns from users' behavior as detected by sensors. Since there is currently no generalized framework for reasoning about activity-aware applications, designers can only rely on the existing systems for guidance. However, these systems often use a custom, domain-specific definition of activity pattern. Consequently the guidelines designers can extract from individual systems are limited to the specific application domains of those applications. In this paper, we introduce five high-level guidelines or commandments for designing activity-aware applications. By considering the issues we outlined in this paper, designers will be able to avoid common mistakes inherent in designing activity-aware applications.