If your undergraduate work is in a field other than urban studies and planning, it is important that you clearly describe your decision to pursue your desired academic and professional path in your Letter of Intent. You may mention how you became interested in this field and what you plan to do with your degree and knowledge. You might consider taking a few USP courses at the graduate level as a post-baccalaureate student to get a sense of what will be expected of you and to demonstrate to the Admissions Committee that you will be a successful graduate student. Also, volunteer activity in planning activities or community organizations is a way of demonstrating an interest in planning.