#1 Be Yourself! Think deeply about who you are and what is important to you and showcase that in the interview. Law firms are businesses where you engage with real-life people, not textbooks.
Do some research on the firm and have some key points you can note that are different to other firms. Prepare a few questions. Be yourself and show your personality.
Be yourself, be ready to describe yourself and who it is you are - this is probably more important and tends to require more thought than the "factual" aspects of your experience, university experiences or grades.
Be energetic, humble, down to earth and show that you are keen to learn and to get involved in the firm (not just professionally, but socially). Don't try to be too perfect - but try to be someone that others would want to work with.
Self-confidence is key. Good grades help. Dress professionally. Be genuine and show interest in the firm.