Guiding principles are central to theory development in physics, especially when there is only limited empirical input available. Here I propose an approach to such principles looking at their heuristic role. I suggest a distinction between two modes of employing scientific principles. Principles of nature make descriptive claims about objects of inquiry, and principles of epistemic action give directives for further research. If a principle is employed as a guiding principle, then its use integrates both modes of employment: guiding principles (...) imply descriptive claims, and they provide directives for further research. By discussing the correspondence principle and the naturalness principle as examples, I explore the consequences for understanding and evaluating current guiding principles in physics. Like principles of nature, guiding principles are evaluated regarding their descriptive implications about the research object. Like principles of epistemic action, guiding principles are evaluated regarding their ability to respond to context-specific needs of the epistemic agent. (shrink)