Ambient lifestyle feedback systems are embedded computer systems designed to motivate changes in a person's lifestyle by reflecting an interpretation of targeted behavior back to the person. Other interactive systems including "serious games" have been applied for the same purpose in areas such as nutrition, health and energy conservation, but they suffer from drawbacks such as inaccurate self-reporting, burdens placed on the user, and lack of effective feedback. Ambient lifestyle feedback systems overcome these challenges by relying on passive observation, calm presentation style and emotionally engaging feedback content. In this paper, we present an ambient lifestyle feedback system concept and provide insights from the design and implementation of two prototype systems, Virtual Aquarium and Mona Lisa Bookshelf. In particular, we discuss the theory and practice of effective feedback design by drawing on elementary behavioral psychology and small-scale user studies. The work is aimed at aiding in the design of ambient persuasive technologies and ambient interaction in general.