A big mistake many of us have made from time to time is to cozy up to a single contact within an organization. Even while I sold millions of dollars of product each month to giants like Boeing and Washington Mutual, I hedged my bet too often by working with only minimal contacts.
Why not focus on just the decision maker? Well, sometimes he or she isn't really the decision maker - they may have told you that, but in fact it may not be true. Or it is true, but in a long sales cycle, things could change - they could take extended leave, they could quit or be let go. Why put all your eggs (your valuable time and expertise) into one basket?
Even if you are working directly with the decsion maker, and he or she is there for the long term, there can be multiple people involved in a larger or more global company purchase. A question I like to ask is, "Who, in addition to yourself, will be involved in this decision". The great thing about this question is that even if your contact is not the decision maker, this question will not make them feel bad (like, "I know YOU'RE not the decision maker, so who is?") and the answer will let you know a lot about who you are dealing with. If the answer includes his or her administrative assistant, that lets you know how important that person is to the decision maker. You'll find out how finance, operations, HR, and other departments are involved.
A good article about the various types of people involved in a purchase decision can be found at the Avidian website.