What's your top tip for doing well at interview?

It's cliché but definitely be yourself and be honest. The interviewer wants to see the true you and you will feel more comfortable. Also if you're relaxed it's easier to have a more casual chat where they can get to know you as a person and not just on a career level.
I would say keep answers clear, concise and end on directly answering the question/giving a result. You want to engage the interviewer by making your answer easy to follow and understand.
In your interview, also try and include something you feel can set you out from other candidates. Something they can remember you specifically by.
And finally, just remember the interviewer wants to get the best our of you, they're on your side! So the situation is never as nerve wracking as you may first think.