Goodluck!! I asked a couple of my friends to meet me before hand and wait with me until they called me in. It really calmed my nerves, especially as one was male and really fit and gave me a kiss - I went in with a big smile on my face!
Also, remember that you will only probably be nervous in the first ten minutes or so, you then just settle into it. I actually forgot sometimes that I was in my viva, it seemed more like a slightly scary chat