Interviews can be stressful, but they can be made even more so by not preparing and making sure you avoid the following.
Allow plenty of time for your journey. Plan to be ten - fifteen minutes early, and figure out the right train or bus to get you there at that time. Then take the one before it.
Make sure you have used the correct address to plan your journey. In a big city, there are sometimes duplicate addresses varying only by postcode. If you encounter any confusion, call the employer to make sure you have the right one.
If the world conspires with traffic accidents or cancelled trains to make you late beyond your control, call the employer and notify them as soon as you are able. Stuff happens, we all know that, and getting in touch will at least show a degree of accountability.
Interviews have two purposes: to find out about you, and to find out if you are suited to the role. Make sure you do your research.
Turning up unprepared is a waste of both your time and the employer's time. We're not just talking about honing your interview techniques (though you should certainly do that). You should also make sure you have with you:
Usually this happens when interviewees try too hard to be funny and charming, or when they become too relaxed during an interview. Don't be lulled by an employer's informal attitude - it can lead you down a dangerous path to using inappropriate language, or telling anecdotes better saved for the pub.
This usually means one of two things - either the interviewee is simply not that interested in the role, or they become so nervous that they freeze up and come across as bored or disinterested. If your nerves start getting the best of you, take a moment to take a deep breath or two. It will help. And remember that it is okay to take your time coming up with answers to the interviewer's questions.
It is also important to recognize that even if you consider the position a stepping stone, mentioning that in an interview may show lack of commitment to your potential employer. It is good to talk about the skills you hope to gain from the position, but if it is a long-term position then you should also be able to demonstrate a commitment to the role.