It may not be the most popular or favorite thing to do, but having the ability to impress adults can be an extremely helpful skill, in school and in the workplace.
First, you want to dress nicely. Adults like to see a clean cut well-dressed young man/young lady. Don't try anything daring or something you think might be scandalous. It will only make them think less of you. Remember, they probably still think of you as children, so cleavage or visible boxers are probably not a good idea, nor are they appropriate.
Next, the first impression is vital. It goes a long way with adults. Be confident and charming, look them in the eye and speak clearly and articulately. If they go for a handshake, don't go for the hug. Make sure your handshake is firm and convincing. You want to convey that you have confidence in yourself. Try not to be awkward or embarrassed. It makes you look bad.
If the adult wants to have a conversation, it will most likely be about college. Be prepared with answers that are longer than two words and that actually engage the adult in conversation. If you know anything about them, ask about it. If you find that you have something in common, all the better! It can be something to talk about and discuss. Joking is fine as long as the jokes or sarcastic remarks are not off-color or insulting to anyone.
Don't be shy about your accomplishments and try to hide them. But at the same time, be humble. If something you've done is brought up, politely answer any questions about it honestly and concisely. If the adults inquire, elaborate. They want to see that you're passionate about something other than video games or shopping. Talk about the things that make you stand out in a good way. That way, the adult can remember you for something unique.
Finally, when you leave, make a polite exit. Don't keep glancing at your watch like you're impatient, but give all your undivided attention to the adult. If you need to leave, drop some hints or say somehthing like, "I really enjoyed meeting you, but I've got to get going, I'm going to be late to ____! Good luck with _____!" Depart with a smile and all will be well.
Now, this definitely sounds like a suck-up to all adults routine. But it works. Adults are older and want to be treated with respect. If you feel like you're imitating your mom or your dad, that's because you probably are. In order to impress adults, you must act like one.