I went to a
college preparatory high school, so I'm a
veritable expert on the matter.
The first, most important principle to keep in mind is that most people have low self-esteem. They naturally assume that they aren't very intelligent, indeed, that they are the dumbest thing to walk the earth since Al Gore. So, the key to appearing smarter than you really are is to magnify that insecurity, rather than to create it.
There are two main categories of effort in doing so. One is appearance, the other is your speech mannerisms.
Appearance: Unlike commonly believed, clothing choice is not of utmost importance; body language is. If you want to seem smarter than you really are, it is necessary to avoid eye contact with people unless absolutely necessary. Eye contact breeds annoying familiarity, which allows other people to view you as a friend and thus as an equal. There could be nothing more detrimental to seeming smarter than you really are. Stare straight forward, eyes aloft or down at the ground. If you choose to keep them aloft, you will appear to be seeing through people, and thus, focused on some abstract goal no one else can comprehend. If you keep them on the ground ahead of you, you will appear to be absorbed in thought and rather intense. Feigned intensity is very impressive for people with low self-esteems.
Walk briskly and take stairs two at a time. Swing your arms rhythmically, as if nothing should dare break the pace. If possible, carry several books with visible signs of aging, or books that are at least obviously from a local university library.
This is not to say that clothing is not important. Quite the contrary. While it is of secondary importance to the carriage of one's body, clothing can add the final conviction in the minds of the weak that you are of superior intellect to them. If you're male, your clothing should be subtle and conservative. Wear belts and loafers. If you're female, your clothing should be subtle and middle-right liberal. Avoid dresses, skirts and heels of any sort unless you're going for a specific office look.
Speech Mannerisms: Follow the teaching of the Tao Te Ching: He who speaks, does not know, and he who knows, does not speak. In other words, suppose A is a person, then A speaks if and only if A does not know. If you are following the Appearance guidelines, people will already assume much more of you than they probably should -- don't mess it up by using a word incorrectly.
But when you do speak, make sure that what you say is well-founded. Monopolize conversations by speaking at length about areas that are your specialty and of which others will be generally ignorant. When the topic should stray from your specialty, speak in sweeping generalities that ultimately bridge between unrelated disciplines -- allowing you to change the subject back to a topic with which you are more well-versed. Doing so impresses the listener into believing you have a grasp of larger, more complex ideas. This works because they do not presume to have sufficient knowledge to debunk your generalities.
Eliminate the usage of prepositions at the end of sentences. "Of which" and "with which" always sound very clever. Take great pains not to split infinitives. Use a vocabulary which is commonly known but little utilized in practical communication. Revive archaisms. Take advantage of the principle of lateral diction.
Lateral diction is the principle by which fairly common words and phrases are replaced by less-used synonyms. Suppose, for example, you wanted to express your dismay at an individual's wardrobe choice. You could say:
"That is so a fashion don't!"
or you can say:
"I have aesthetic qualms about donning a gray shirt with khaki slacks."
Your audience, appropriately, will interpret the second statement as pretentious and thus slightly humorous. The humor you are exploiting is an effect of lateral diction and is not commonly used by people with low self-esteem. You will thus seem even more intelligent in their estimation.
It might be noted that this particular course of action is anti-social in the extreme. It is. Having friends is not about seeming smarter than you really are. If you want friends, consider another node: How to seem dumber than you really are.