Flexibility is recognized as a key feature in structuring software, and many architectures have been designed to that effect. However, they often come with performance and code size overhead, resulting in a flexibility vs. efficiency dilemma. The source of inefficiency in software architectures can be identified in the data and control integration of components, because flexibility is present not only at the design level but also in the implementation. We propose the use of program specialization in software engineering as a systematic way to improve performance and in some cases, to reduce program size. In particular, we advocate the use of partial evaluation, which is an automatic technique to produce efficient, specialized instances of generic programs. We study several representative, flexible mechanisms found in software architectures: selective broadcast, pattern matching, interpreters, layers, and generic libraries. We show how partial evaluation can systematically be applied in order to optimize those mechanisms.