Here's an increasingly important one - be sure about whether there might be 2 - Mrs. Wannascapes before you ask about a Mr. Wannascape (or vice versa).
To tell the truth, I've never done much to ensure both spouses, partners, whatever are there. They usually are. But there could be simple questions like:
Does this day/time for the appointment fit everyone's schedule?
(I thought of a couple others, but they're all derivatives of the one above, so I deleted them)
As for meeting with the clients at the initial meeting, there's a mindset I have, and an approach I try to take...I think I might save part of this for 'The Cabin' forum (part of the Private Gardens) when I open that area up. But the part I'll share here is, in general I find that when it comes to things aesthetic, the woman is in charge - she usually picked the drapes, the fixtures, the pictures, and everything else that has to do with appearance in the house. Because what we do is in large part aesthetic, I focus on her much more than the man. For the man, so long as I can show that we know what we're doing, we can do what we're describing for this project, can do it on time and on budget, he seems to be happy. His eyes start to glaze when I talk about color wheels, textures, warmth of colors, etc. That's also the time that the woman's eyes light up.
Not coincidentally, a few minutes later the man usually finds something important in another part of the house that he must tend to.
Asking the questions Paul mentions are great - getting them to start thinking about and picturing what is really going to go into their yard and how they'll use it is important.