Don't go off topic. Keep your brain on the things you are good at.
Consider your responses.
Do not be worried to say you don't know a response to questions.
Interact, perhaps, with the firm's twitter profile.
Don't stress yourself by thinking about negative thoughts.
Put on a nice suit. Be smart and neat.
Don't forget your body language like making eye contact, your posture, not fidgeting and not mumbling.
Advice for Job-hunters:
"The search for a new job starts with you writing down your ideals, specialities, prowess, and objectives. How are you to get a role if you are not clear of what you require a job to be and what you are able to do for a potential firm?"
It may be helpful to have:
A logical mind and strong work ethic
Ability to adapt to change and work effectively as part of a team