[quote=Goldmarble]I have to agree with Magnum, you need to dress somewhat fittingly for the type of work you're being interviewed for. 
[/quote]

Pretty much this. Wear the clothes you would wear if you were going to be your own boss at this job -- so like, if you're applying to McDonald's or Gamestop or retail, dress like a manager; if you're applying for construction, dress like a foreman (minus safety gear).

Generally you want to demonstrate just a couple of things at your interview:

1. [b]You have your shit together[/b] (you can dress the part, you're not on drugs, you show up on time and you're not a lunatic). Playing the 'I need this job' card hurts you here.
2. [b]You're serious about working *specifically* here.[/b] Nobody wants to spend three months training you up and then suddenly you're on to better things.
3. [b]You're not going to be a problem-employee.[/b] Problem-employees come in many shapes and sizes; they're seeking this potential in you, so they can weed you out. Don't let them.
4. [b]You'll give them more than you take away.[/b] Jobs are business, business is money, money has to make sense. Be a hard working contributor, know your role, etc. The resume can really help on this one -- highlight things you've done, teams you've been on, etc. that went far and accomplished a lot -- the more involved you were, the better.

In terms of 'how should I act in the interview itself,' don't be **too** worried. You might be nervous -- everyone is nervous, nervous isn't bad. The person interviewing you *probably* isn't your future boss -- it's someone who's seen a ton of interviews and can sort out the normal people from the assholes. That's their job. So.... don't be an asshole, pretty much, and you've done the best you can.