Here's a few things I picked up while looking for new people over the past few weeks:
0. Re-read the job posting details, and get clues out of it. See if you missed any of the requirements or hints when you sent out your application letter.
1. Be prepared, bring your portfolio, CV, transcripts, etc. Pen is a must, too. Pencil is optional.
2. Always expect to be tested verbally, logically, technically, and abusively ,)
3. Dress neatly, but don't overdress if that's not the usual you. We can see through anyway.
4. Know when to answer and when to ask a question. Do not give "I don't know what to ask" when you're asked "Any questions?"
5. If possible, learn about the company. At the very least, you should know the company website or the product / project they're working on (you'd be surprised at how many people who "didn't remember" the URL of our company website).
6. Did you remember to ask the interviewer what to expect for your interview? Things like: Who you should be meeting, how long the interview would take, and if there's anything you should prepare for? Things like that scores you some points since it shows your preparation.
7. If you're going to end up late (even if it's only for 5 minutes) or not show up, give the interviewer a call at least 15 minutes before the appointment.
8. Be ready to answer seemingly stupid and innocent questions like "What is your favourite pet?" or "Do you know the difference between web designer and web developer?" or "What do you think about ad XYZ?". They're there for a reason.
Here's a test for you: Why did I start my item numbering at zero? ,)
Add that to the list of items mentioned by D0ct0rteeth, colinmack, and nomade, and you should stand out from the rest. And of course, like hanschien mentioned: have fun during the interview. Don't get too anxious or we'll get bored easily.
By the way, there IS a difference between a web designer and a web developer.
-stndn.