I read an article a few weeks back that told me it takes 17 times of seeing someone again to diminish the first impression. If this is true than a lot of people need to really refine their day to day appearance; me included.
It is crazy how powerful a first impression is. Even if you are not face to face with a person.
Sometimes, I can tell how strong a candidate will be just by the voicemail they have set up on their phone. If it is professional and well spoken, I take that as a solid first impression. My co-workers and I often laugh about the fact that a lot of people forget to change their email address names, or "ring back tones" on their cell phones. It really is not the best first impression, especially on the top of a resume, when the person's email reads "email@example.com". If you are job searching, and put your resume on sites such as Monster or CareerBuilder, it is well advised to change your email address to something more subtle.
I personally also remember a good handshake. It is almost a let down when I go to meet a very qualified candidate for the first time, they are dressed professional, yet barely shake my hand. As a job seeker, I believe it is important to make a name for yourself right from the start. Separate yourself from others first thing! A handshake could definitely set you apart, but if it's a poor handshake, it will set you apart in a negative way.
Lastly, I think smiling can be a huge factor in a first impression. When I go to meet someone for the first time, I always give a warm smile and make them feel comfortable. It is also easy to see how they will react back, and then tell if they will be a good fit for our client. It is good to act serious in a job interview, but no smiles usually means too much business and no relationship can be built. There is a lot more that could be said about first impressions, but I will stop here for now. Moral of this story: control the things you can control while job searching. Practice your handshake, and update your voicemail!