I'm going to agree with knightofdelta, above. In most cases the HBS interview should feel more like a conversation - would you practice a conversation in front of a mirror?
My advice? Find a current HBS student or an HBS alumnus (ahem) that you know, and just talk to them. Don't tell them you want to do a mock interview, but do mention that you have an interview coming up. You should have a nice conversation, where many of the topics from the interview will come up. It will help you feel more comfortable in your own skin, and will hopefully reduce some of the anxiety you may be feeling.
Good luck, everyone! Feel free to drop by the chat room if you need to vent