The same way you prepare for any other interview: RELAX.
If you go into an interview stressed and nervous, guess what the person or persons interviewing you will think? "Wow, he's nervous." That may be enough to cast doubt in your capabilities and abilities in their mind.
Look at an interview as a conversation, with the topic being what you have done and what you can do. Don't be cocky, but be confident. Put together your portfolio that showcases your best work… but don't volunteer to show it – wait to be asked. The best interview I had was one that I didn't have to show any of my work at all – the people interviewing me could tell that I could do the job because I told them what I would do if I got it.
Which is a key thing – besides getting your design samples in order, decide how you will actually DO the job if you get it it. It's easier if you are applying for a job you've done before, of course… prepare an answer, and volunteer it if the interviewer(s) don't ask you directly. It helps, and demonstrates your capabilities and that you have thought beyond just getting the job.
Finally, listen and react. If you are applying for a UX job, soft skills are very important, and you need to demonstrate these in an interview – because you'll be using them in your job. A lot.