The "weak means strong" principle
The one body-language principle that holds up over time: non-professional players unconsciously try to hide the strength of their hand and therefore broadcast the opposite. Someone who sighs, hesitates and says "fine, let\'s try" usually holds a strong hand. Someone sitting upright, staring you down and betting with exaggerated confidence is often bluffing. It is not an iron law, but it is the strongest observed tendency in live games.