1. Understand that it is what you make of it.
You might not get a salary for your work, but you are getting paid with experience. This means you might have to recognize the opportunity to gain that experience yourself, and what you learn might not be what you expect. You might encounter an internship where all you do is get coffee. But don't let that kind of internship keep you from benefiting from the experience. Use it as an opportunity to test drive the industry. Consider the size of the department you're part of. What size is the company? Do you like working in this kind of environment? Do you like the pace? Was your experience what you expected? All of these factors weigh in your decision about your future career. You should consider your internship to be a very long job interview, but you should also remember that you are interviewing the company and the industry too. Getting the most out of your internship involves more than just learning how to accomplish the tasks you are assigned.
2. Learn how to work.
Unfortunately, learning how to work requires on-the-job training. There is no class in college called "Working 101". At work, you will find social mores, business etiquette and workplace politics not mentioned in any textbook. You must interact differently with colleagues at different levels. You must be independent and keep track of your tasks. You must know how to follow-up. These are different responsibilities from those you have in college. Everything that happens during your internship represents an opportunity to learn how to navigate in the corporate world. Pay attention, and when you get a paying gig, you'll have a much easier time.
3. Listen carefully and ask questions.
When your supervisor delegates a task to you, he expects you to own it so that he can forget about it himself. Listen to his instructions to make sure that you know what he wants accomplished. Also, make sure you know how he wants it accomplished. Don't finish a task only to find out that there were expectations about it being done a certain way. If you don't find this out in advance, your success in completing the task will be overshadowed by the fact that you didn't use the "right" method. Learn how to ask probing questions that get at the heart of your responsibilities. Don't be afraid to ask those questions. Your internship (and your future job) will depend on information that you learn through listening and asking questions.
4. Dress appropriately.
Every workplace has standards for dress, whether they are stated or not. The best way to find out what is appropriate is to ask your HR contact or your supervisor. They will be able to give you the best description. Beyond that, take a look around, see what the staff wears and emulate that. And one other thing: I know it's summertime, but those cut-off shorts and flip-flops are for the beach. If don't dress like a professional, you won't be treated like one, and you won't get the respect you deserve.
Of course you hope that your internship will lead to a job offer, but remember that the connections you make every day might lead to future employment. To create those connections, you must network. Part of what an internship provides is access to a network. The people you meet are working in the industry that you dream of getting into. Get to know them. Find out how they got their jobs. Find out what they look for in employees. Get to know the other interns too; they may be helpful in making connections. Create a profile on LinkedIn to help keep track of everyone. And stay in touch. If you go back to college and forget about the people you met at your internship, you risk wasting the network your internship gave you access to. Nurture that network and you'll be surprised by what might happen.
The quality of your internship will be determined by how you approach it and whether you got the most out of it that you could. If you keep these points in mind, you will get much more out of your experience than just a bullet point on your resume. Good luck!