Think positively. There is no need to get stressed before an interview. The people interviewing you just want to get to know your skills and your personality.
Handle rejection. Just because you didn’t get the job, don’t take it personally. There are literally hundreds of possible reasons why you may not have been suitable for the position, and it doesn’t mean you won’t be right for the next one.
Don’t be critical of current or previous employers. Even if you have legitimate reasons for being critical, interviewers will be wary if you overly chastise others. Focus on how you have overcome difficult situations instead and the actions you took to resolve any form of conflict. If you can’t do this, then it’s better not to mention it at all.