If you have to make a decision where you can't be certain, let 99.999% = 100%.

This forces you to take a position, and not become paralyzed by the outside chance that the extremely unlikely scenario might happen: if you're throwing a dart at a dartboard, it's true that you might hit a bullseye on the first try, but you can be nearly certain that you won't.

When someone is trying to manipulate you for their own gain, one of the things they will do is -withhold key information-. Because you can't be absolutely certain that what they're saying isn't true, your instinct will go along with how they want you to behave, because of the nagging "What if...?" scenario that is created: what if I go to hell if I don't believe in this doctrine, even though I am almost totally certain that its teachings don't hold water? What if I give up the chance to become a millionaire if I don't give my money to this firm that produces seemingly impossible returns on investment? What if this person who has been caught lying to me in the past really does have my best intentions at heart this time, even though their story doesn't add up?

I'm not saying that there aren't exceptions to this rule, but it has helped me to detect when I might be putting myself at risk when putting my trust into a situation that could potentially have very negative repercussions.