Now that I'm here, I forgot why I came

Not too long ago, I was driving two salesguys to a meeting. Because one of them had flown in late, I was hauling (in my Dodge Caravan). The guy who’d come in late asked me if I could participate in a call he was getting on, as he needed a technical resource. I said, sure. He said, “I just sent you the dial-in info.” I pointed out to him that I was doing a bit over the speed limit and, most important, was frigging DRIVING. I handed the other guy the phone and asked him to dial me in. Bluetooth in place for hands-free conversation, I was now partially listening, since I tend to pay more attention to the road than anything else when I’m DRIVING. We began discussing some contract language. The guy leaned forward and whispered, “I just sent you the docs the lawyer is talking about, in case you have any opinions.”
Temporarily stunned, I put myself on mute and said, “In case you haven’t noticed, I’m still freaking DRIVING.” Only a salesguy would expect you to read word docs on your Blackberry WHILE you’re doing better than 70 on the tollway.
My mistake was in not setting expectations. I guess, at the outset of our journey, I should have made it clear: “Your life, and mine, are in my hands. To this end, I will focus on staying between the lines and avoiding contact with other speeding metal crates on the road, so that we may reach our destination and then return safely home.”
This is the diplomatic version of, “Hey dumbass, I’m DRIVING.”
Expectations should be set at the beginning of any transaction, and occasionally restated.
Years ago, I started a cycle with a large beverage company. We put together a plan for basic auth and SSO for a single division. But then all the subsidiaries started trying to jump on board. All these companies, with all their own portals and directories, and they wanted a giant umbrella to put them all under. I told him until I was blue in the face, stop trying to do it all in one big shot, because he would never succeed. Stick to the original plan, I reminded him.
And more recently, I dealt with a large agri-science company that kept changing the plan; swapping authentication schemes, swapping authentication directory, changing their SSO strategy, widening, then narrowing, then again widening the scope of which groups of users would be included in the first pass; adding, then deleting, federation as part of the scope; changing the repository for security questions; and so on
When in doubt, pick something small. Get basic auth and password management (meaning policies and reset) up and running. SSO, or at least reduced sign-on. A common user directory, at least for the users in the first scope. Show that you can do that much. Then build from there. Make sure you’re not building something in the first phase that can’t be reused. If something crops up, tell the cropper-upper they have to wait for Phase 2, unless they’re the guys with the doughnuts. Don’t boil the ocean, just a single cup of coffee.