If you're playing cards or have a tiny white lie to tell in person, keeping a completely straight face isn't necessarily the best tactic. A recent study suggests a slight smile or anything "emotionally positive" is what really throws people off.
In a study of poker mechanics and how they relate to emotional "tells", psychologists around the world found that when players tried to rely on face reading (most often for medium-value gambles, poker fans), their reaction times and correct guesses went down the most when the opposing player had some kind of positive face on. Maybe it speaks to the difficulty of keeping a truly neutral face as opposed to running with a straight-up smile, but it's worth thinking about the next time you're in need of a little wiggle-room.
Human Wagering Behavior Depends on Opponents' Faces [PLoS ONE via NYTimes]