Kev posted some good advice on this topic a while back, so I am quoting him here...thanks Kev! Here it is:
Hiring and DX managers want someone who is demonstrably friendly and eager to work, very thankful for the chance to interview, doesn't arrive with a sense of entitlement, confident that he is right for the position but meek enough to show a certain malleability to conform with how they do things. It's a tricky act to balance; confidence and entitlement, for all jobs.
Dispatch offices all throughout this country are spattered with cocky blowhards who are quick to assume dominance, denigrate others, may know their stuff, but are caustic elements to the workplace who have nothing positive on a personal level to contribute (as are most professions where a college degree is not a requirement). Some dispatchers are HR's worst nightmare. I wouldn't say these people are in the majority, but they're there and managers are acutely aware that they don't want to add another to the mix.
Don't give anyone a reason to think that you aren't humbly grateful and gracious for the mere opportunity for an interview. Show your inherent enthusiasm for the field of aviation and your excitement to get started. If you know you're the right guy for the gig then relax, make eye contact frequently with every single person in that room, smile and joke a little. Have many questions ready to ask them, especially a few very technical ones for the dispatch manager. Be proactive if you think the interview went well and you are still interested in the company. Say just that at the end, or find a better way to express it. Assume you'll never talk to them again (because if you call them or send e-mails you may very well not), so seal the deal there and tell them how well it went for you and how interested you are in the company, and how you look forward to speaking with them again soon. Write a hand-written thank you card that very night and mail it to them.
Once you have a face-to-face interview the onus is all on you to make it happen and show them that you're the right person for the job, and in a lot of ways it comes right down to your personality, how you conduct yourself, and the touch you have to maintain a proactive stance to scream "hire me!" without being too loud. They're trying to get a good read on you; are you a good person? Are you enthusiastic? Are you going to whine at work? Are you professional? How will you work with others? How excited are you to learn? etc.
Good luck! When all else fails, I say channel President Clinton and you'll do nicely.
I would add to this - dress nicely (but don't wear a tuxedo), and have copies of paperwork/personal information with you in case you need to fill out a job application, or if they need another copy of your resume or whatever. I normally will wear a sport jacket, tie and slacks with a long-sleeve dress shirt. Most offices are now business casual but it never hurts to be a little bit more formal (unless they specify what type of dress they'd like you to wear at the interview in advance...I've had one airline do that also.) Also, in this day and age, I think thank-you emails are as good a thank-you card (plus they give you a chance to show you are at least minimally computer literate) and you're sure to get the message there more quickly than by snail mail - but that is just my opinion.