There are several key things to keep in mind with considering your first impression as a candidate for a new position: Attire. Are you wearing the appropriate attire for the situation? For most companies, business attire is expected at the interview. First impressions can make or break.
Preparedness. Have you researched the company? The industry? Trends? Solutions? Have you reached out to your "network" at the company (or at least spent time up-front discussing with the recruiter before the interview)? Show up having done "the" research prior to an interview. Arrive with purpose and an objective in mind.
Non-Verbals. Eye contact is key. Fidgeting can be distracting in an interview and may demonstrate your discomfort with the situation. Make sure to pay attention to the non-verbals of those you meet in an interview as these are often indicative of behaviors.
Smile. Positive Attitude Changes Everything (PACE) and a negative-nellie is easily recognized. Don't share the doom-and-gloom in your life (in an interview) as it is a major turn-off to someone you have just met.
Share relevant examples. If you are interviewing for a consulting position, share project stories and experiences. If you are interviewing for a marketing position, share marketing experiences and examples. Be selective in what you share as the interviewer is looking to "connect" ideas and expectations for the position and your sharing relevant content is key. Make it easy for the interviewer to fit your experience in the guard-rails of the role.
Hold off on Salary Questions. Many times, the interviewee feels that things are going so well in an interview and that asking questions about salary and benefits is appropriate. Hold off on these questions until you receive a formal offer or the hiring company initiates the conversation. Going to "$$" too early may set you up for rejection as the hiring manager may think your first / only focus is money.
Do not ask "How did you think I did?" while at the interview. A major pet peeve of many hiring managers is when the candidate asks "how things went" at the end of an interview. Other related questions include "Do you think I will be back for the next round?", "Anything I could have done better?", "What areas of my resume are weakest?", etc. This can put the hiring manager in an uncomfortable situation and potentially set false expectations of "hope" for a candidate that will not be invited back for Round 2.
Most of this is common sense. Blocking and tackling. And if you stay on top of your game, you will set yourself apart from those who do not prepare.