Excellent topic! My attitude varies from whether I'm playing "rookies" or the intermediates. Against the former, I'm often overly cocky, & make foolish mistakes. Against the latter, I'm often too wound up & prone to panic, & make foolish mistakes.
But, as of the latest Forum/APC party, I've discovered a great leveller - BEER! Yes, a comfortable, manageable level of intoxication takes the edge off, & allows me to relax, but still keep my wits.
Kids, don't try this at home (until yer of legal age!)