Microgrids have gained a lot of interest under the smart grid vision demanding systematic procedures for their optimal construction and performance enhancement. By defining a new probabilistic index, this paper presents approaches for clustering active distribution systems into a set of microgrids with optimized reliability and supply-adequacy indices. After proposing two different methods for optimally designing the microgrids, as two corrective actions to improve the defined index, the advantages of installing distributed reactive sources (DRSs) and distributed energy storage resources (DESRs) are investigated. The problem formulation and solution algorithms are presented in this paper. The well-known PG&E 69-bus distribution system is selected as a test case and through several sensitivity studies, the robustness of the design and the effect of interconnecting different amounts of DRSs' and DESRs' capacities on the defined index are investigated. It is found that by properly designing the system and integrating the DRSs and DESRs, the index can be improved from 23% in the base system without distributed generators (DGs) to 77% in the basic microgrid design with distributed generators and to 93% considering all resources.