As a last futile, desperate effort to prevent a call, many players will start to spread a bluff (or a weak hand) on the table. You can actually elicit this tell by reaching for your chips and then gauging the reaction of a player who has bet into you. If your opponent starts to spread her hand prematurely, you can be pretty certain that you can profitably make the call. Use this tactic when you're leaning toward passing. If the bettor starts to spread her hand, continue to make the call smoothly. Otherwise, pass. Often you can salvage an entire pot because a player will apply this fairly common mannerism in an attempt to prevent you from betting.
