These are all excellent suggestions. Additionally, something that may help you to "be yourself" and to smile is to try to interject some appropriate humor into the interview.
For instance, you could say something like "I had a great time writing my biology comic strip for the school newspaper. Yes, I know I'm a terrible nerd, but I love being one!" and then make a big grin. Or, if they comment on the fact that you seem serious (one of my interviewers said this to me, which wicked surprised me since I'm usually laid back), you can say something like, "well, I automatically get a little stiff when I'm being choked by this rediculous tie."
I mean, humor is SO relative and it's hard to know which jokes still seem professional and cute and which just seem out of place, but it's SO important to try to laugh during the interview, and make your interviewers laugh with you. It helps to break the tension!