Here are some resources and references for your resume and cover letters. Consult http://www.northeastern.edu/careers/jobs-internships/resumes/ for some great advice and references for your co-op search!
General Tips for Resumes
Make sure to include most relevant information. Is it a policy research co-op? Include past experience in research, policy course-work, and interests/skills.
Edit and polish. Ensure there are NO mistakes on anything you submit in an application. There is nothing worse than applying for a detail-oriented job with simple formatting/grammar mistakes, employers will immediately discard it.
Use ACTION verbs to emphasize past experience. Click here to view a page with some great examples.