Be yourself.

“…style is intrinsic and private, like…voice or gesture, partly a matter of inheritance, partly of cultivation.  It is more than a pattern of expression.  It is the pattern of the soul.”

-Maurice Valency

Think of language as haberdashery: you have a closet full of clothes for every occasion. Your clothing choice expresses yourself in a particular context, for a particular audience.  In the same way, barely aware of it (or not aware at all), you change your speech to what you think (though there are no conscious thinking processes) will be effective for a particular situation and audience.