The first step towards being taken seriously by friends and strangers is to take yourself seriously. Sport your greys. If you don't have any, dye your hair salt-and-pepper. Unless you are under 25. If you are under 25, nobody should take you seriously. Just so that you can do whatever the hell you want, and say, “Dude, I'm young, don't take me seriously.” If you are over 25, do everything you do, with a dead serious face. You need to consider yourself to be the most important person alive and the only person whose opinion matters. If you don't believe it, nobody else will.
Speak less. Smile lesser. The only time you must smile is out of condescension. The more you talk, the more you expose yourself and the more you allow people to judge you. Keep to yourself as if everyone around is a lesser mortal, and rather hang out with people in your league. On occasions that you speak or write or need to give a speech, make sure that the ratio of regular words to words-that-require-listener-to-refer-to-a-dictionary is 5:1. Always introduce yourself as a mysterious recluse who likes to keep to himself till, lest they see through your arrogance. Don't add everyone on Facebook, let them add you. And accept very few. On Twitter, just follow a chosen few such as Obama, CNN and one or two of your famous friends who talk to you.
Never offer unsolicited advice. Give it at a premium, in terms of cash or at least, beer. And then, stamp your authority every time you are asked for opinion or advice. Authority could be in terms of your age, experience, exposure, qualification, awards won, positions held or once held. When you are speaking to those who don't know you all that well, make sure you subtly talk about: “When I was...”, and end with “That was a long time ago, anyway”. This shows that you not only did something cool but also that you've been there and done that ages ago. Make sure you give out visiting cards with all your degrees and positions held, to everyone who meets you.
You need a professionally-shot photograph with a moderate smile. Too wide and you'll look like an idiot. Too thin, you'll look like a serial killer. Don't make it too goofy, it needs to make you look intelligent. This picture must go with any articles your friends in the media can do on you. Else, use that as the header on your blog.
Always ensure that your wisdom can be defended by actions of Gods in mythology ical epics or by great thinkers and philosophers. ***Do you know about the man who built a management school by re-interpreting managing principles from the Mahabharatha and sued anyone who called it shady? Learn from him, Don't allow people to talk trash about you; sue the hell out of them. Freedom of expression is overrated.