I agree that you shouldn't try to oversell yourself, but if you're a confident person, this should come across. I think the key is to be yourself as much as possible and to be able to demonstrate that you're the type of person who would be easy to work next to and get along with. Being dynamic/energetic is one thing, but being "over the top" is quite another. If they didn't think that you would be capable of doing the job, they wouldn't be interviewing you, so what they're really trying to figure out is what kind of person you are and whether you would fit in with their organization. This is especially true of summer internships; they're not expecting you to be working there for the next 30 years, so coming up with the next "breakthrough" vision for the company's future will not be your immmediate task.
Learn as much as you possibly can about the company, as this shows an interest/eagerness, but don't try to "demonstrate" this knowledge in a way that is forced or doesn't fit into the conversation. What seems to work is a matter of fact approach, where they will immediately understand that you're demonstrating your preparation for the interview but won't be rolling their eyes and thinking, "Here we go again, another kid who knows everything..." Try something like, "Oh ya, I was reading about that on your website... but what I was wondering is..." in response to a comment or quesiton that an interviewer poses. This way you are showing respect for the flow of the conversation without trying to force anything. But always have a couple of company specific questions ready to fire off when they ask you, "So what other questions do you have?" The more you can show that you understand their business envoronment, and the issues they face, the better off you'll be.
Interviewing is kind of like going on a first date, but the advantage that you have is that you've seen the girl and know that you're interested in her. She hasn't seen you yet but has heard good things about you (i.e., like your resume that the company sees) so she's willing to give you at least one chance based on a recommendation from someone she trusts (i.e., like your references). You've got an entire life of stories to tell (some interesting and funny, and some that you would rather not tell her quite yet), but on that first date, she's not wanting you to "impress" her so much as for you to be yourself so she can see if she feels a connection to you as a person. So be yourself, but be your "good" self!