As much as possible, let the designer share the experience of your customers. Let them use your products, find out for themselves what is good and bad, and talk to existing users.
Definine the boundaries for your product, but also accomodate very broad, innovative thinking early on. This is often when the best ideas appear. The designer should then use a systematic method to select the lead concepts; help them by having a clear idea of what you want the product to be, and who you intend to buy it.