Robert Cialdini's six principles, first published in Influence (1984) after years of him working undercover in sales, fundraising, and marketing roles to study persuasion in the wild, describe recurring levers that shift compliance: reciprocity (we feel obligated to return favors), commitment/consistency (we act in line with prior public commitments), social proof (we look to others' behavior under uncertainty), authority (we defer to perceived expertise), liking (we say yes more to people we like), and scarcity (limited availability increases perceived value).
Understanding these isn't just useful for persuading others — it's a defense. Recognizing 'this offer is manufacturing urgency through artificial scarcity' or 'this salesperson gave me something small specifically to trigger reciprocity' lets you evaluate the actual merits of a decision separately from the influence tactic layered on top of it. Cialdini's own later research found that simply naming the principle out loud to someone using it on you — 'I notice you're creating urgency here' — often neutralizes it immediately, because manipulative influence tends to work best when it operates below conscious awareness.