Great question. From the viewpoint of an evil rhetorician, the short answer is "build a more convincing argument." You're already asking that, so it seems what you need is techniques to improve your argument and, in turn, your persuasiveness. I've found that one of the best ways to get buy-in is to get the client to come to your own decision without feeling like it was fed to him/her.
In its most effective form, this technique is pretty much the cartoon trope where Bugs Bunny and Elmer argue, then Bugs takes Elmer's position, Elmer unknowingly takes Bugs' and ends up doing what Bugs wanted all along.
One way to do this is through a best-of-class "switch-er-oo" examples. An anecdote:
We had a corporate web site redesign and the client stakeholders were having troubles getting away from their current site (a thing perfectly reasonable in 2001). Mockups weren't performing well in reviews, so we took a step back and tried a different approach.
We performed a "study" of their direct and tangential competitors, pulling front pages together and highlighting mistakes and inefficiencies. Then, instead of showing them positive examples from their industry we showed them successes in best-of-breed from other industries. We used BMW's USA site and Apple's main site. While most designers will yawn at the choices, it was an effective argument. The stakeholders were much more receptive to our design afterwards, being able to say "We're going to be the BMW/Apple of _."
It was also effective because they were the ones to decide to take the Apple/BMW route. While we were just showing good / bad design, what they saw was
- Their industry makes bad design choices
- If they make similar choices they will just be following their industry
- If they make different choices they can lead their industry
- They can be like Apple / BMW (what can I say, sex sells :) )
They were the ones to say "Try it like those last couple," and even though we'd been doing that before, they were the ones to "think of it" so we had our buy-in.
Being the apt business persons they are, your clients will be focused on their own industry, trends, competitors, etc. They will see what others are doing and it will shape the schemata in their epistemologies. It's difficult to change that preconception, meaning you need to do so in a way that breaks it and reshapes it, without them thinking you're assaulting their world view. This can be accomplished by presenting information (which to you is related but to them will be unrelated) and creating the opportunity for them to discover the connection and make the leap. This way, their "corporate web site" schema will change and you'll be the person there, waiting to take new suggestions.
Just make sure never to take credit for their discoveries--Bugs always got in trouble because the rabbit always let Elmer know he'd been "duped." Your victory is that your design wins and that your clients are a little better informed.