Every year fewer students are electing to take university level science courses, particularly physics. This situation has led universities and employers to try and encourage more students into science subjects through the development of numerous science outreach initiatives such as guest lectures and summer schools. Much of this work is of an ad-hoc nature, with little understanding of the underlying motivations and conditions that guide students' subject selection. Therefore, we have analysed a range of literature sources to develop a simple model for school student engagement that can help guide university-initiated science outreach programmes. This model takes into account factors such as the different life stages of the student, the myriad influences that affect career decisions, and resource availability. As part of this work we also present an overview of the research around increasing progression into higher education. As experienced practitioners and researchers in science outreach we have developed an easy-to-use set of guidelines that are applicable in the real-world situation of limited budgets, time and staff resources.