Authentic equals fire!
There's a lot of different ways to approach this, I think that the first and probably the simplest way is to basically get out of your own way. LOL. That means to stop listening to what you sound like, because when you listen to what you sound like it then detracts from the authenticity of the story or the copy that you're trying to tell or sell.
The direction seems simple enough, “Please sound authentic and real”. But is it simple? There are a few techniques you can use to make sure you stay focused on the story of the script, who you’re talking to, and sounding like you’re not reading from a page.