School improvement plans are strategic documents most schools complete on an annual basis. Research on school improvement planning highlights that high-quality plans contribute to student achievement gains, but many plans are of poor quality. Principals serve in a critical role within the school improvement process. In this article, we review research on decision-making and biases to provide a set of recommendations to improve the school improvement planning process. Specifically, we highlight how a set of biases can contribute to flawed planning that reduces the likelihood that the school improvement plan will be comprehensive, strategic, and useful. We also include a set of definitions and "planning guardrails" principals can use when developing a school improvement team and facilitating the school improvement planning process.