1. Tie up loose ends. Make sure any projects you have been assigned have been completed. This is by far the most important, because if one of your projects fails after you leave, there's a good chance that that is how you will be remembered. Hopefully you have been updating a list of assigned tasks throughout the internship as McKenzie Koch suggested in an earlier post, otherwise piece one together and make sure everything is complete. Also, it may be helpful to include a brief overview of what you contributed in a thank-you note to your supervisors; this will be helpful if they offer a reference.
2. Get a performance assessment. You spent a whole summer trying your best in the job, so go ahead and ask for a report card. What does your immediate supervisor think your strengths and weaknesses are? How can you fix those weaknesses? Which careers might best foster your strengths?
3. Refresh your resume. Don't wait until January to update your resume with what you did this summer. Chances are you will not remember the nuances of your tasks, so go ahead and ask your supervisors while you still can. Additionally, depending on your relationship, your supervisor may even be willing to review your resume.
4. Express gratitude. Make sure to thank your supervisors. Do it regardless of whether your experience was positive or negative. And after thanking them in person, send a thank-you e-mail from your personal e-mail address (I have also seen interns write out thank-you cards). Among those who reply, it may be a good idea to add them to your LinkedIn account, as business e-mails often change.
5. Stay in touch. When saying goodbye to your supervisors, employees you've worked with and those who have simply enjoyed your company, feel free to ask for a business card or an e-mail address. A great way to stay in touch is to shoot an e-mail including what you have been up to every few months. Maintaining these relationships is important as your mentors and other co-workers can provide strong guidance to learning and entering the corporate world.
All basic stuff, but so important to remember, especially in this economy and job market.