You never know when you are going to be tapped for an interview, especially when you are an active member of a professional organization. When you get invited to interview, you need to be prepared with a stunning resume and cover letter.
Your resume should always be prefaced by a cover letter that states your intentions and briefly highlights your interest in the company. Your resume is essential because it is your personal marketing package. Start working on your resume early.
You will find that it is much easier to simply update your resume periodically than it is to have to start from scratch when you try to get that first interview!