Practice makes perfect that's the only way to do that, but even if a player is good at hiding his expression and his feelings, experienced poker players are also good at reading your body language, so it's not only your facial expression but also your body language that you need to hide for you be competitive in poker games.
Being afraid of giving away takes during poker games is real issue. That is probably why at the moment is the best time to play poker on casino. Wearing a face mask will hide most our expression, if on top of that we wear a scarf, hat and glasses it should be impossible for anybody to read us. But if we do that than it is very likely that other players will do so too. Being able to show expressions is a good thing in my opinion because we can look excited when actually having a bad hand. As long as we can control our emotions we can make good use of them. Then again when playing online all these points are invalid.