That puts the burden on you to make certain that those two seconds really count. The only way to do that is to prepare ahead of time.
These tricks may help.
1. Keep Yourself Fit
Your energy level is dependent upon your overall level of health. If you tire easily, rest assured you're likely to look tired--especially under the stress of an initial meeting. And if you look tired, other people will make the snap decision that you're too tired to get the job done.
This does not mean that you need to be a bodybuilder or Hollywood thin. But you must be healthy enough to look alert, capable, and interested.
2. Research the Culture
Different industries have different norms about what's appropriate in terms of personal appearance and meeting behavior. For example, wearing an Armani suit to a meeting with a programmer is simply inviting silent ridicule. Similarly, different regions of the country (or the world, for that matter) have different norms. Women who wear even slightly sexy outfits can send the wrong message to managers from the Middle East, for instance. Find out what's expected before you meet.
3. Send Clear Signals
Your semiotics are the signals that your appearance immediately communicates to other people. People make snap judgments based on clothes, accessories, and more: watches, jewelry, briefcases, makeup, skin tone, facial expression, and so forth.
As far as is practical, make sure you are consciously creating a set of visual signals that is most likely to communicate that you're the kind of person that's it's appropriate to do business with.
4. Create the Best Greeting
There are three parts to your greeting: your smile, your words, and your handshake (or your bow, in some parts of the world). These vary according to situation.
Research is key: For example, some cultures view toothy grins as vulgar. Your words should be appropriately formal (or informal). Handshakes should generally be firm rather than crushing or limp. (And bowing is very specific to the situation.) Do your homework!
5. Know Your Agenda
A huge part of a first impression hinges on whether you seem confident in yourself and what you've got to offer. People sense at gut level whether you're prepared for the subsequent meeting or just planning to wing it.
That's why it's important to know what you want to accomplish at the meeting and be ready to accomplish it. Being thoroughly prepared creates a poise that silently communicates you're credible and reliable.
6. Rehearse Your Entrance
Now that you've put all the parts together, do a dress rehearsal of how you'll enter the room--or, if it's the other person who's entering, how you'll stand and deliver your greeting. Rehearse it enough times so that it all becomes second nature, rather than merely rote memorization. If possible, practice with a colleague and get feedback.
7. Measure and Adjust
Since first impressions are so crucial to success, you'll want to track the results of your efforts. After each initial meeting, note the response of the people you've just met. Pay attention to facial expressions, statements, and subsequent behavior.
If the meeting also included a trusted colleague, ask for feedback. Did the first impression you made help or hinder? Where is there room for improvement?