You aren't always going to be the smartest person in every group you take part in but you can always get the edge by looking smarter than everyone around you. And if people are truly honest, most would admit that it is more important to give others the impression that you are smarter than they are than actually being smarter. But whatever your motive, looking smarter has it's serious advantages in life.
Dress the part. Smart people know they don't have to resort to showing off sexy legs, big muscles or their keen sense for exotic looking fashion trends to impress someone when they have greater intelligence. They want to be taken seriously for their smarts, not their body. You should always dress for the professional job you dream of one day achieving. Don't be the only one who can picture you taking on that coveted role, make others picture you in that position as well.
Cut and style your hair in a no-nonsense sort of way. Keep to clean lines and simple styling. Always have your hair well groomed and off your face. Take extra care to keep your hair healthy. Also keep makeup to a bare minimum.
Wear glasses if you have them. Even if you only pull them out to read contracts or other papers shown to you. Wearing them shows a serious side to you. Putting them on to read something of interest to someone else, in front of them, gives them the perceived notion that your smarter because, after all, smart people would take them seriously. It's human nature to prefer people who seem to prefer you.
Exude confidence. Even if you need to muster up the illusion of being confident, it is worth the effort. Confidence draws people to you and makes them realize that there must be more to you than meets the eye. They will instinctively be curious to learn more about you and take what you have to share seriously. Confidence and intelligence are often perceived as one in the same. Make eye contact, use good posture, shake hands firmly and walk with purpose.
Be interested and ask intelligent questions to probe further. Always be, or at least appear to be, interested in learning more about the subject at hand. Never appear to be a know-it-all. Trying to give the impression that you are already an expert in a given field with nothing more to learn gives people the impression that you are going to be both hard to get along with and a braggart who really won't have much to contribute due to your lack of open-mindness to learning more. Intelligent people know that learning only stops when we die and not a second beforehand.
Make eye to eye contact with everyone you meet. This draws on the same vein as the confidence issue. Eye contact takes a degree of boldness, something intelligent people must have in order to become intelligent to begin with.
Be observant. You do not always need to remark or react but do 'get caught' being observant as often as possible. This also includes being observant of other people's reactions, feelings and needs.