Often when we try to influence someone, we try to do so in a "rational" manner by using stats and logic, because surely if they could only see the "facts" then there is no other conclusion.
What we fail to see is that "facts" rarely convince or persuade anyone, they really just help reassure ourselves that we are right. The best way to convince someone of something? You have to tap into their emotions.
Don't read that as manipulate. What it basically means is that you have to help them care. Which is why it is so hard to understand why we sometimes use argumentative language that only gets people defensive. If the person on the other side of your cause is guarded because of you, your cause is already lost. Check out this observation from Made To Stick (67):
Emotions are elegantly tuned to help us deal with critical situations. They prepare us for different ways of acting and thinking. We've all heard that anger prepares us to fight and fear prepares us to flee. The linkages between emotion and behavior can be more subtle, though. For instance, a secondary effect of being angry, which was recently discovered by researchers, is that we become more certain of our judgments. When we're angry, we know we're right, as anyone who has been in a relationship can attest.
Humor opens us up to possibilities. Two-way conversation provides an avenue for people to find bridges into other possibilities. Persuasion is not a battle to be won, but a joining of arms in a cause. Be strong in your beliefs and wise with your words.