Know what you're applying for, have the necessary back up documents for the position applying for.
Presentable, casual dress but not shoddy
Know your capabilities and have your credentials ready as well as your references. You should also bring your resume just incase. You are selling yourself so don't say anything negative, always upsell. If you don't know how to do something, don't say I don't know how, say " I have limited experience in that area, however, I am a very quick learner and I am eager to do so"