They'll just try to please you.
Work to understand your customers' problems
Ask them questions about their problems instead of setting them up to tell you what you want to hear.
Be genuinely curious about what goes into their problem
Have their ever sought out a solution? Why or why not?
Customers usually have no idea what they want
That's the point. You want to understand their problems at their core, and them form the solution yourself.
Be careful not to delude yourself
It's very easy to ask someone if your idea is "good". It's a lot harder to genuinely feel out what the market needs.