In my personal life I tend to be quick with a smile and a joke. I really enjoy trying to make people laugh and put them at ease. But at work, it's a very different story - most of the time anyway. When dealing with people in a professional setting one must try to have control of the perception they give off. Effective business people have to be able to instill confidence and trust in those they interact with. I don't always succeed in doing this since my natural tendency is to be silly and obnoxious.
So when I'm at work, I go into what I affectionately call "Kirk mode". This is particularly true if I'm talking to external business associates. "Kirk mode" is named after a good friend of mine who, seemingly naturally, has most of the qualities one looks for in a successful professional. Since most people don't have access to my friend, here are 10 things that I've found are absolutely critical to polishing your professional image to achieve success in business:
#1 Limit your humor. Use jokes and humor very sparingly. Watch a movie and the leader of a group has a good balance between jovialness and seriousness. Those who crack jokes constantly rarely achieve the kind of respect they may want.
#2 Slow Smiles. Smiling at people is important. But you want people to feel like they've earned it. Carry this through phone conversations -- they may not be able to see your smile but they can probably sense it.
#3 Eye contact. Eye contact is essential. Give it to whomever you're speaking to. It will take some practice to know where to draw the line between maintaining eye contact and staring.
#4 Never tease. Winners don't have fun at other people's expense. They naturally give off the impression of confidence. Confident people don't need to tease others for a quick laugh.
#5 Talk less, listen more. Some people I know talk as if they simply like hearing their own voices. And when they aren't talking, they give off the impression that they're merely thinking of what they want to say next. You are almost always better off being a listener. Speak enough to keep the conversation going (i.e. don't give the impression you're quiet because you're shy but rather because you're interested in what they're saying).
#6 Posture. Posture. Posture. Slouching isn't just bad for your back, it conveys a lack of confidence. People's first impression of you often comes before you even speak. Stand up straight and you'll find that people will react more favorably towards you.
#7 Be Positive. Avoid criticizing others or talking negatively about things when you're first interacting with someone. This is the one I struggle with the most and even as I'm criticizing something in front of my coworkers my inner voice is saying "Stop doing that now." Being positive is important and adds weight to when you do have to be negative on something.
#8 Be loyal. Success in business (and in personal life) requires other people wanting you to be successful. And one way to do that is to convey through word and deed to others that you are working towards their success and that you will be loyal to them. Truly see yourself as an agent to help other people succeed. Your friends and coworkers aren't your competitors, they're your partners. Help them succeed and you will find success follows you.
#9 Control your anger. Never let someone get your goat. Anger and frustration always give off the impression of weakness and inconsistency to others. I struggle with this a lot, particularly on-line since stupid people anger and frustrate me. 
Hold off on sending that email or biting back at that co-worker. Angry, mean, nasty people will sabotage themselves, they don't need your help -- especially if it comes at your expense.
#10 Don't fidget. Control that urge to scratch your nose, itch your hair, and just plain fidget when talking to people.
You can't fake this stuff, not easily anyway. You have to truly be genuine. To do that, you have to see these things as desirable traits to have in yourself and try to grow yourself into that. When I go home and I'm around friends, particularly ones I've known a long time, the class clown version of me comes to the fore out of habit -- plus I tend to just like to be that way too.
Ultimately it's about knowing which aspects of your personality to present to people and knowing when each is appropriate. Good luck!
Hold off on sending that email or biting back at that co-worker. Angry, mean, nasty people will sabotage themselves, they don't need your help -- especially if it comes at your expense.