Knowing the Opposition
A big step toward failing to persuade is taken whenever we fail to consider the arguments of our opponents. When I say "consider: I don't mean "scramble to discredit." I mean seriously consider to determine what has merit and what does not.
Doing so is not only important from the standpoint of doing the right thing. It is also important because if serious consideration has not been given, then later on some of those opposing arguments may turn out to have merit. When that happens, the revelation can be fatal to your own position.
One of the signs of a truly educated person is healthy skepticism toward information and experts. I confess that I've grown to be highly skeptical of the status of "expert" regardless of whether or not I agree with the person's opinion. When it comes to sorting out the truth or the best course of action, no one deserves automatic deference.
Raise an eyebrow and know the counter-arguments.