Suggestions on how to control facial expressions (more)?

I have no trouble with keeping my mouth shut or controlling my body language. But I have a lot of trouble controlling my facial expressions, they're just so...obvious. Like if I'm scared, uncomfortable, or disgusted people can tell. To just quit making them sounds like it should be so easy, but it isn't, at least for me. The problem is the facial expressions happen before I even consciously realize I'm making them, it's like a reflex. I really want to control them and know how to keep my face stoic and expressionless for the sake of being polite.