I know a couple of high school students who learned the lesson about making good impressions the hard way. One day they discovered a substitute teacher in their chemistry class and immediately started high-fiving each other and roaring, "Party, party!" While the substitute introduced himself to the class, these two boys rapped out loud at their desks. When the sub turned on a video for the class, they started telling jokes. They kept talking even after the sub told them to be quiet.

Ten minutes into class, the sub sent the two boys down to the principal's office. But that wasn't the worst of it. The next month, the chemistry teacher resigned and the sub was hired full-time to teach the class.

"We wished we would have acted better when we first met the sub," the two boys admitted. "He had us pegged as troublemakers, so he was extra hard on us. If we even said a word while he was talking, we got written up. If we needed extra time on an assignment, he wouldn't give it to us. We were on his bad side, and he wasn't going to let us get away with anything."

Their story underscores a vital point: First impressions do matter. Communication experts caution that we have less than 20 seconds of interacting with strangers before they form an opinion of us. In that short amount of time, decisions are made such as whether to hire somebody, distrust someone or become a person's friend. In the case of these two students, they got labeled as being disrespectful and unruly. Once an opinion is formed, it's very difficult to alter. That's why it's important to give each new encounter your best shot.

As a high school or college student, you will be meeting new teachers and students, college recruiters and potential employers in the months and years ahead. You may be invited to parties where you hardly know anyone, or visit out-of-town friends or relatives and be introduced to their friends. All these people will form an opinion about you—based not only on what you say but also on your facial expressions, mannerisms, attitude and appearance.

Here are 10 ways to give them a good first impression:

1. Dress appropriately

The clothes you choose should be tidy, clean, stylish and modest. Avoid sexually provocative clothing. "A girl might think she's just wearing something fashionable, but if it's too tight or skimpy, it sets her up for being viewed as a sexual object and for guys to make inappropriate advances towards her," warns Gary Hill, Ph.D., a clinical psychologist with the Family Institute at Northwestern University.

Guys, too, need to make sure they're not giving the wrong impression by the way they dress. In particular, avoid the super low-rise pants with your underwear showing or shirts with immoral messages (such as about getting high or drunk). "You may think these clothes send the message to your peers that you're cool, but it sends a very bad message to adults," Hill says.

2. Speak clearly

Do you say things like "gimme," "goin'," "wanna" and "Watchadoin'"? Do you mumble? Do your words run together? If so, you are suffering from "lazy lips." People will be more likely to take what you say seriously if you speak clearly and distinctly.

Listen to how fast you talk. Some say the best rate for talking is between 120 and 160 words a minute. You might want to record some of your conversations, so you can figure out how many words you speak per minute, on average. If it's way above 160, make a conscious effort to slow down. "Speed talkers" often communicate little and appear nervous.

3. Make good eye contact

When you're having a conversation with someone, look directly at the person and make good eye contact. Try for eye contact that is gentle and relaxed. You don't want to turn it into a stare-down, nor do you want it to be forced.

This is especially important when going on interviews or meeting with college recruiters. If you're not afraid to look the interviewer in the eyes, you will stand out from the crowd. You will appear comfortable, confident and friendly.

4. Smile sincerely

A smile can do wonders if it's the right kind of smile. If you force a smile because you think it is expected of you or because you are just trying to be polite, you'll appear insincere. The best smiles are a lot more than just moving some facial muscles. They come from inside. You smile because you're happy to meet or be with someone. With a genuine smile, you will certainly make the people around you feel good.

5. Unplug yourself from technology

Put your iPod, iPhone, PSP and other handheld electronic devices aside when you're meeting new people. True, it may seem "acceptable" to your friends to listen to your iPod while you wait on customers, or to text at a dinner party. Yet "to people your parents' age and older, that seems like you are shutting them out and aren't interested in interacting with them," observes Patricia Leavy, Ph.D., associate professor of sociology at Stonehill College in Easton, Massachusetts.

The people you're with deserve your full attention. Anything less and they won't feel like they really matter to you.

6. Don't talk nonstop

Unless you are on a job interview, you should avoid talking on and on about yourself. I was recently on a cruise, and during meals I sat with a lot of people I didn't know. One woman talked about herself nonstop, mostly bragging about career successes and how well-traveled she was. Only rarely did anyone else get a chance to speak. After a couple of days, I noticed people avoiding her table. No one wants to be around someone who is self-absorbed.

Try to see the person you are with as the center of the conversation. Resist the temptation to tell all your funny stories or share everything you know about a topic. Avoid hogging the spotlight. Instead, try to come up with questions to draw other people out and get them talking. Most people enjoy sharing their perspectives, experiences and stories.

7. Listen actively

Just letting other people talk is not enough. Give whoever is speaking your full attention. This is how you show respect, courtesy and interest. Nod during the conversation and ask pertinent questions to let the speaker know you're listening.

Give positive verbal cues like, "Hmmm, that's fascinating!" "What happened next?" and "How did you feel about that?" Don't gaze in another direction, interrupt the speaker, twiddle your thumbs or keep checking your watch. If you do, you will give the impression that you are bored and disinterested in what he or she is saying.

8. Monitor your body language

Use your body language to project confidence and approachability. Sit and walk uprightly, without being rigid. When you walk up to a group of people, look up and smile as you greet them. If you're sitting across from someone who is talking to you, lean toward the person slightly to show you are interested in what's being said.

Avoid crossing your arms or legs. This makes you seem nervous or defensive. Don't put your hands on your hips or clasp your hands behind your head. That conveys disapproval or arrogance to others. If you're seated, don't wiggle around too much, tap your feet or play with your hair. Excessive movement signals nervousness, impatience or boredom. If you slouch in your chair, or walk into a room with your shoulders hunched and your head down, you will come across as insecure.

9. Be polite and courteous

Always be polite, considerate and courteous toward the people you meet. Make it a habit to say "please" and "thank you." If your teacher asks you to do something, do it without challenging him or her. If you're doing something in class that's annoying other people, stop the behavior.

10. Cultivate an easygoing, positive attitude

Nobody likes being around complainers and critics or people who are too outspoken and confrontational. I remember meeting my college resident assistant during my freshman year. She spent about 20 minutes making negative comments about the dorm and telling me how she "wasn't afraid to tell people off."

She may have had a lot of good qualities, but I never found out. After that first encounter, I avoided her as much as possible. I didn't want to spend time with someone who was focused on the negative and all too ready to pick a fight.

Hold back any tendencies to criticize, complain or grumble. Instead, be approachable and positive, and look for good aspects even in the face of difficult situations. Having an upbeat attitude is another way to instantly attract other people to you.

These 10 tips will help you make better connections with the people you meet. Of course, you may not always handle every encounter perfectly. That's okay. I had a friend who once spilled coffee on her blouse on the way to an interview, and she still got the job. Her new boss could see what kind of person she was deep down inside.

That's what's most important. If you are genuinely concerned about other people, it will be obvious by what you say and do. If your heart is in the right place, others will like you, and you will make a good impression. VT