Here's what my teacher once said (on Career Guidance Day):
1. Never sit without the permission of the employer.
2. Greet him/her.
3. Always wear a CONFIDENT smile. (Note: All of them are expecting you to be nervous that's why you should not show it to them. Be unique. ;))
4. Look at him/her in the eyes. If you can, never leave them. 
5. In answering the questions, BE SPECIFIC. Just hit right into the bush! Don't murmur about how, honest and loyal you are. Tell them what your past experiences are. Example: For the past 3 years, I already earned P...., 00.  in the blah,blah,blah Company.