How can I become less trusting of strangers?

So today a stranger stopped me and told me that he had locked his keys in his car and wanted some cab fare to get to his house so he could grab his spare. He took down my info so he could get to me and pay me back, and said he'd be back in a couple of hours. Sure enough, those couple of hours came and went, and I never saw the money I lent him again. Is it wrong that I'm so trusting of complete strangers? And how can I stop getting suckered in like this, since it's not the first time someone has taken advantage of my generosity?