Even if you are sweating under the collar, and spent hours planning whatever it is you are about to do, project an image of a sort of "devil may care" confidence and the feeling that you have everything well in hand and people will flock to you.
It helps if you have a good(and unique) sense of humor. Myself, I tend to lead towards the "holier than thou" sense of sarcasm, and it seems to get loads of laughs because I also let them know that its all an act, and that I really do care about their companionship.
What was the old saying? I may be paraphrasing but it is something along the lines of the best way to get friends is to become one.