if u only started recently and are yet not used to it, there is actually not much to change, u just need some adjustment and reasoning. i mean, just dont treat everybody nicely, not esp strangers or non-acquaintances, dont make the nice face the default one. wear a mask that says tell me who u are and i'll tell ya how i treat you. BUT, if u are already used to being nice and cant just stop grinning at strangers just try to be serious for a while manually and you will get used to it. always remind urself to not look like a goof:)