It helps to have taken and done well in courses on statistics and economics, particularly microeconomics. Courses in planning and related to planning, e.g., public policy, environmental studies, urban studies, etc., are also helpful. Many people enter the field of planning from seemingly unrelated disciplines. If so, consider strengthening your application by taking graduate-level courses in planning at PSU (or elsewhere) and/or gaining some experience in the field. This could include volunteering for a community or planning organization.