Ask them a lot of questions and really listen to what they have to say. Also talk to people who aren't currently your customers to find out about their pebbles. But there can be gold in what everyone else considers to be garbage. I once volunteered to develop a smoking policy for a hospital when the health promotion staff wouldn't touch it. It solved a touchy problem for the hospital and was one of the most satisfying projects of my career.
Organizations are especially vulnerable to that kind of complacency.
Don't ever take your existing products or services for granted. Always be on the lookout for ways to improve them. Now that we've got overnight delivery, faxes and e-mail, you're lucky if they give you till the top of the hour. Most of us have tunnel vision. We get so used to the way things are that we don't see how they could be.