My sense is that the most rational argument is the one that gets you closest to your true goal. The theme of my most persuasive arguments is usually something like, "Once we sort things out I bet we'll see that we're really in substantial agreement." Dale Carnagiesque or not, to me rhetoric doesn't have to be a "dark art," full of manipulation and cant. Instead, I try to give respect and keep in mind what I really need to take away from the transaction. I also conceptualize what my erstwhile opponent needs and why. This way, minds may yet meet and all parties have a line of social retreat.