Being persuasive is often a function of being (or at least appearing) trustworthy. The more people trust you the more likely they are to listen to your words and do what you want. For that purpose, the HAIL method can guide you.
Sound consultant Julian Treasure explains the four principles of the HAIL method: Honesty, Authenticity, Integrity and Love. While it might sound cheesy, living out these four ideas all have a direct effect on how trustworthy you come off and, as a result, how persuasive you are:
The H, honesty, of course, being true in what you say, being straight and clear. The A is authenticity, just being yourself. A friend of mine described it as standing in your own truth, which I think is a lovely way to put it. The I is integrity, being your word, actually doing what you say, and being somebody people can trust. And the L is love. I don't mean romantic love, but I do mean wishing people well, for two reasons. First of all, I think absolute honesty may not be what we want. I mean, my goodness, you look ugly this morning. Perhaps that's not necessary. Tempered with love, of course, honesty is a great thing. But also, if you're really wishing somebody well, it's very hard to judge them at the same time. I'm not even sure you can do those two things simultaneously.
Of course, this isn't revolutionary news. The more authentic and honest you are, the more people will be willing to trust you. That's how trust works. But running through your conversations, speech and behaviour through this filter can help keep you on the right track.