Students often make mistakes in physics courses and are expected to identify, correct, and learn from their mistakes, usually with some assistance from an instructor, textbook, or fellow students. This aid may come in many forms, such as problem solutions that are given to a class, tutoring to an individual student, or a peer discussion among several students. However, in each case a student relies upon an external agent in order to determine whether, and how, her work is mistaken. Consequently, the student's learning process is largely contingent upon the availability and quality of external evaluating agents. One may suspect that if a student developed the ability to evaluate her own work, her dependence on external agents could be moderated and result in an enhancement of her learning. This paper presents the results of a study investigating the impact of novel activities that aim to teach students when, why, and how to use the strategies of unit analysis and special-case analysis. The data indicate that it is possible to help students dramatically improve their understanding of each strategy, and that this has a significant impact on problem-solving performance.