Be on time. Make sure you and your supervisor are clear on your starting date and the hours you are expected to work. Set a schedule and stick to it.
Know the dress code. If you're not sure how to dress at first, err on the conservative side. It's better to be too dressed up than too casual. Better yet call your supervisor and ASK before you start your internship.
If you run out of work, ask for more. Don't wait for someone to notice. Be aware of what projects others are working on, and when something looks interesting, offer to help if you need something to do.
If you don't understand something, ask questions.
Be prepared to do a certain amount of menial work and do it with a smile.
Don't get involved in office politics. Don't gossip.
Watch your language.
Avoid making personal phone calls if at all possible.
Keep a positive attitude and remember what your mother taught you about saying please and thank you.
If you are having any problems at all, remember that our office stays open all summer. Call.