Have questions prepared.
Do not bring up any topic unless you are prepared for a full in depth discussion of the subject.
If you list something as a hobby, you better know it like the back of your hand.
If you mention socialized medicine, know what countries have it and the advantages and disadvantages of it.
Be honest. Don't hedge or cover up.
Be positive. Turn weaknesses into strengths where possible.
Think before you speak. Don't give the impression that the answer is rehearsed.
Don't ramble. Give enough information to answer the question and them wait. Don't let silence get to you.
If you are unsure of what an interviewer is asking, ask for clarification.
Don't try to second guess. Request more specificity.
Poor communication skills
Not being informed
Using "pull" or name dropping to gain an advantage
Low moral standards
Lack of interest