The Professor Is In recently wrote an article about Networking as a Postac, which inspired this post.
One of the best things you could do as a graduate applicant is to find a list of the major conferences in your area (ei – ABCT) in the fall. Then look at the abstracts, and pick one in which a lot of the mentors with whom you are applying to work with will be attending. Invest the $ (perhaps you can share a room with someone, or crash with a friend) and attend this conference.
During the conference, you should attend the presentations given by your potential future mentors. After each presentation is over, go up the potential mentor and introduce yourself. Mention that you will be applying to work with them. Many will ask you to email them your CV. If your CV is otherwise good, this will express interest in the program and general initiative, as well as ensure that your potential mentor looks out for your application.