Earning the right to have an opinion
If I'm going to listen to your opinion (and it's really just an opinion no matter how forcefully you say it), I have to believe two things:
First, I have to believe that you know what you're talking about. I have to believe that you've done this before, that you've rolled up your sleeves and battled with this kind of problem and not just watched from the sidelines. People who have really done something sound very different from those who passively experienced it.
Second, I have to believe that you have my best interests at heart. There's no easy way to do this. You have to invest time and effort in understanding my interests, and in doing all the things it takes for me to trust you.
Only then will you have my full support.