Wireless sensor network technologies are expected to be a key technology to support various innovative applications in the future ubiquitous computing environment. So far, the main stream cf sensor network research has been focused on simulation-based development cf battery-aware communication protocols. However, when we apply sensor network technologies to practical applications such as environmental monitoring or factory automation, we will find a lot cf practical problems that are not considered in simulation-based approach. We believe that finding and solving such practical problems are quite important for deploying sensor network technologies beyond laboratory use. From this point cf view, we have been trying to find such problems through various implementations cf sensor network system for several years. This paper describes our opinions cf designing sensor network system and introduces our approaches for developing practical systems.