A colleague of mine is fond of saying, "You are always selling." It's a frame of mind that doesn't always come easy to me, but I seem to find more and more evidence that he's correct. There are very few things I do where I'm not trying to influence people.
One of the standard books in the selling space is Neil Rackham's SPIN Selling (my review from several years ago), where the basic idea is to understand the Situation and Problems faced by the client. Then get a deeper understanding of the Implications of those problems for their business (sometimes it is obvious to them, sometimes the extent of the implications isn't quite as clear). And finally, get to the Needs-Payoff of resolving the problems. This same concept is expanded in Rackham's Major Account Sales Strategy, published in 1989, a year after SPIN Selling. The difference with "major accounts" is that the burn time from first introduction to "the sale" can be much longer, and as a result, the SPIN approach gains more nuance. But the basic flavor still exists: understand what is going on and what keeps people up a night. With larger accounts, the "implications" and "needs-payoff" becomes much more interesting due to division-of-labor and other factors that makes it difficult for people inside the organization to see all the implications and possible benefits (and hazards) of a change.
Major Account Sales Strategy breaks the selling process into its steps: Recognition of Needs -> Evaluation of Options -> Resolution of Concerns. And each of these have some separate elements as well. The point Rackham makes, is that the approach one makes depends on the phase. You don't push a sale when the customer doesn't think there is a problem. And you don't keep hammering on the nature of the problem when the customer is interested in finalizing the deal. This lines up very nicely with the Theory of Constraints approach to "buy in" - tying the conversation to where the people are in agreement that there is a problem, agreement on the direction of the solution and agreement on how to implement the solution. (In fact, the people who have recommended I read this are very familiar with Theory of Constraints.)
Another colleague described Major Account Sales Strategy as describing a process of successively "asking for permission" to continue. I'm not sure I read that the first time through, but looking at my notes and the high-level flow, this "permission" is essentially tied with the levels of buy-in as one moves through the cycle Rackham describes. Rather than permission, I saw a lot of discussion around influencing their thinking: helping people appreciate the extent of the problem; influencing what variables are important (that align with our offering); and a number of other elements.
Along with the basic framework of these phases of major sales, the book is full of examples and studies that show where these ideas make sense (and where in people struggling when they operate outside this structure). Rackham draws from his own experience and that of many sales people with whom he has worked. In fact, the next business book I picked up (Goldratt's Necessary but Not Sufficient) starts with a story that could have been added to Rackham's book: the sales people work to influence the target's criteria on what a good solution looks like.
The book ends with a story and a useful summary quote: "Effective strategy isn't about grand design, and it isn't about clever tricks. It's about a thorough understanding of your customers and the concerns which they have at each phase of a sale."