Many organizations depend on the functionality of mission-critical legacy software and the continued maintenance of this software is vital. Legacy software is defined here as software that contains no testing suite, is often foreign to the developer performing the maintenance, lacks meaningful documentation, and over time, has become difficult to maintain. Because of these flaws, developers frequently make modifications without a formal plan. This type of legacy software maintenance is problematic and often results in a reduction of software quality--for example, more defects, "code smells" and "code rot", high levels of brittleness, degradation of conceptual integrity, and bolt-on modifications. General guidelines for legacy software maintenance were developed in prior work, but none of these guidelines were evaluated during actual maintenance tasks. This dissertation contributes to the corpus of legacy software maintenance research by presenting a novel, practical, legacy software maintenance heuristic named "Heuristic for Improving Software Quality (HISQ)", and by evaluating the effectiveness of the heuristic on software quality through an action research study. The study showed that an experimental group of developers using HISQ improved software quality for a set of legacy software maintenance tasks when compared to a control group of developers. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]