General rule ... never show
Exceptions:
Show when it will confuse your opponents as to what cards you are playing/betting/raising
Show when you aren't getting action, and you think it will entice future calls
Heads up --- I show a lot more heads up. Heads up is such a mind game as opposed to a card game. If I feel it will give me an advantage by misdirecting my opponent, I'll show.
See Ya On The Felt !
