How can I became a great Key Account Manager?

In any organisation's supply chain, three important things are needed: A.
Flow of funds B.
Flow (or movement) of goods C.
Flow of Information.
'A' is Management's & Sales' responsibility and 'B' is Operations' responsibility.
A Key Account Manager cannot directly control 'A' & 'B'.
However, a successful KAM is one who keeps information flowing (C) to all stakeholders AT ALL TIMES, and this becomes even more critical when problems arise with 'A' & 'B'.
Sounds simple.
But this is the toughest part because Information is not tangible.
And, "that which cannot be measured, cannot be managed".
Management of 'C' will have a significant impact on'A' & 'B' In a nut-shell, a great Key Account Manager is one, who gathers information and uses it for the benefit for all stakeholders by increasing revenue for the customer and the company.

Key Account Manager is a lot more than a salesman.
A great Key Account Manager is a conceptual and analytical problem solver who identify the business improvement opportunities and transform solutions to generate revenue for both, his account and the company he works for.
He can handle multitasking that is to build average accounts into key accounts by generating more revenue as well as retaining the existing business.
Well, it needs a lot more discussion and I just mentioned a bit up.
More but different ideas will definitely help you and me too.