Start off on the right foot.
Suit pressed, jacket on when entering. "Relax, take your jacket off" is what you're probably going to hear next. Unless the company specifically states "no ties", doing this shows you care.
Have a copy of your CV, résumé, and applicable certificate(s) handy.
Be prepared to talk about yourself, practice beforehand. You want to come off confident in abilities to contribute, not cocky. Looking in a mirror and practicing is uncomfortable as hell, but it helps.
Review any applicable course material as it relates to the job. Be prepared to ask them a few questions. 135 carriers are not required to use 121 dx'ers, so your job description might differ from a 121 operation.
Don't be a •. Manners and posture go a long way. When someone else enters the room, STAND UP and shake his/her hand. People notice this stuff. There's a lot more, but I gotta go. Good luck!