Here are two scenarios for you to ponder.
You walk into a job interview. Your shirt is unbuttoned, you've got a grease stain on your cheek, you've trekked mud in on the carpet, you're wearing a pair of cast-off running shoes you found in a trash bin and your breath reeks of garlic. It's also obvious from your bloodshot eyes that you haven't slept in several days, and judging by the smell emanating from your clothes you may have spent the time drinking, smoking up or both.
You walk into a job interview. You're wearing a clean, freshly-pressed shirt and jacket combo with a well-adjusted tie and some nice slacks to match. Your shoes are shined, your hair is trim and your breath is an exquisite as a field of fresh flowers.
Which of these two do you think will give the better first impression? It seems pretty obvious, and it is: number two would get the job over number one in a heartbeat, almost regardless of the first's superior qualifications. First impressions mean everything when we meet a new person, and considering how quickly they're formed we need to make sure they're good, because a first impression will determine how that person acts towards you for the duration of your relationship. (Even if it only lasts a few minutes before they kick you out of their office.)
Yet most cases of first impressions won't be so easy to judge. Indeed it takes much smaller signs than the first scenario to rub a person the wrong way. Little personality quirks, clothes they don't like, an unshaven face on an otherwise clean body; all these things, along with hundreds of others, can make for a bad first impression with virtually anybody. Your physical appearance and how you act dictates what kind of person you are, and whether or not they try to shake it off later. The new person you're meeting will peg you as a certain type right away. And once established, it's hard to get rid of that first impression and replace it with something else.
The key, then, is to put your best foot forward when meeting a new person, regardless of who they are or under what circumstances you're meeting them. Be friendly. Be courteous. Show them the kind of respect most people crave. You only have one chance to make a good first impression - and it'll last forever, even if you convince them that you are a different person over time. And even if you start out nice and eventually reveal yourself to be crummy, you can take solace in the knowledge that that person will give you a few second chances just to try and bring out that original personality.