Write a Good Personal Statement
Whether you are applying for a summer-research internship (REU), graduate school, or a career, do not underestimate the importance of your personal statement. It is quite literally the key to opening the door of opportunity.
Dr. Celia Elliott at the University of Illinois has written a primer on writing your personal statement (PDF) for graduate school, but the lessons are just as applicable to research opportunities, internships and jobs.
Some students have found this document to be very useful: A Handbook for Students Applying for Scholarships and Graduate Study written by Joe Schall at the Penn State University.