It actually doesn't take much practice to develop a natural-looking demeanour at the table, regardless of whether you have just made the crown jewels or seen all your dreams go up in smoke. This demeanour may be highly animated or completely motionless (or anything in-between), just so long as it doesn't give away the strength of your hand.
If you play online you don't need to keep a poker face at all. When you hit a big hand you can jump up and down, call over your wife and kids to admire your brilliant play, whatever you like. Your opponents won't be any the wiser. In cyberspace, everyone has the perfect poker face!