So you need to dress appropriately for the interview. You should always dress in professional attire even if the job happens to be a blue collar job. The fact that you have taken the time to make yourself presentable will definitely impress the people interviewing you. In case the actual job attire is casual, you can be a bit less formal after you get the job.
When you step in and greet the interviewer, look the person directly in the eye. Be friendly as well as confident during this first impression. Also remember to smile. In case the interviewer extends his hand, provide a nice handshake which is firm as well as confident. In case your hands get clammy and moist under stress, remember to wipe your hands before meeting the interviewer.
You need to be confident in yourself, but definitely not arrogant. Act as if this is just your job to get and that after the interview, you will be offered the job since there is no better candidate than you. But do not overdo this, as you may come off looking and sounding arrogant. This may not appeal to the interviewer.
Always carry a copy of your resume with you when you go for the interview. Now the interviewer may refer to something on your resume and in case you do not remember what you wrote, you can always use this copy as a reference. Also have a copy of the application which you had filled out so that you can use that also as a reference.
Do your homework on the company before you go for the interview. Find out as much as possible about the company. You can visit their website, talk to their current as well as former employees. Read anything that you can find about the company.
Practice answering various interview questions. There are numerous books that cover this and have ample suggested ways to answer. Get any of these books and go through the various questions and how you can answer all of them. Even the Internet is filled with sites which will guide you through the possible questions that may be asked. When you answer a question, always try to demonstrate how your skills will benefit the company you want to work with.