Don't overlook anyone
Rookie and veteran salespeople alike are often guilty of focusing in on the "buyer" while neglecting individuals who have the ability to kill a deal in the blink of an eye.
Source: Sales/performance coach Kelly Riggs
In fact, there are times when those individuals kill the deal simply because the salesperson ignored them, not just because they didn't address their needs or concerns about the deal.
Rule of thumb: If you don't know who all can influence a decision, assume everyone can until you know better.