1) Letters of Recommendation/ References
Before you leave, let your site supervisor know that you would appreciate a letter of recommendation. Or else, ask the site supervisor if you can put her/his name down for references. A good recommendation (either verbal or written) from a site supervisor can carry you a long way in terms of finding a job.
2) Stay in Touch
Even though there may not be any job openings at the internship site directly after you complete your internship, job openings may appear soon after you leave. By staying in touch with the organization, you will more likely hear about upcoming jobs. Furthermore, because your site supervisor is networked, s/he may know of job openings in other places. So consider calling your site supervisor once a week or month and keep up to date on the organization.