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All employers look for why you bring value to the company. Too many applicants focus on their needs and not on the companies. Study the companies history, track record, achievements and any big time employees that may work for the firm.
You must also be able to pin point what it is you like about Marketing in order for them to get a feel for your long term employment. If you're just a graduate with a marketing degree, but lack a certain amount of intrinsic motivation toward marketing/PR/advertising, then you'll stick out like a sore thumb.
Wear a blue suit. I know it sounds funny, but the color blue instills your ability to work with a team and be a team member.
Talk about what value you bring to the company.
Talk about your biggest interest in Marketing and what you wish to do with the experience you gain from the interviewing company.
Think of a great idea for a marketing campaign for their company so they know you've put serious thought into applying for their job. Don't kill yourself on this but take it serious.
In the end what we're all really looking for is a mentor in what we're truly interested in. Perhaps you should add that as a reason for applying. The fact that you feel you'll get the best on the job training from them.
If marketing is something that you're really interested in, then I would volunteer or even do pro bono work to build your resume.
Hope this helps.