This is especially important when dealing with potential clients. Can't risk putting them off and losing that relationship! Wait til you get to know them, go to the bar and have a few drinks first, see if they have the same humor as you, and then let loose and joke around freely. It also helps to not have a negative or dark sense of humor or personality in general. More people have one than not, but they'll also be quicker to turn on you, so the long-term risk is still too high.