I would suggest to just be yourself. Though this thread really did help me go through my interviews. Now I've got a few job offers to consider.

Here's someting that I would like to share from my experience:

After a few questions...
Interviewer: What is your weakness? (in a serious tone)
Me: I'm a hopeless shopaholic! (you can also say you're a hopeless workaholic... )
Interviewer: What item do you shop for exactly? (in an interested tone)
Me: Shoes! I've got a size 3 feet (the interviewer gave me a shocked look. I know! hahaha) which is a challenge for me when it comes to shoes so I kinda like to go hunting for shoes that can actually fit me which can sometimes take all day! But the end result is worth it when I finally find shoes that fit my tiny feet.
Interviewer: So your weakness is shoes. (She said it in a unbelievable tone while laughing)
and then the interview went back to a more serious question.

p/s the interviewer is a woman. It helps.

What i'm trying to say here is just be yourself during the interview (though it does help to practice a bit).