The iterative design process requires designers to be willing to make significant changes to the design. If you are “married to” your initial ideas, and unwilling to make more than small changes, the value of prototyping and playtesting will be small.
Making significant changes to a design—especially in reaction to outside feedback—is risky. If you don’t have a clear, strong design values prototyping and playtesting will push your design around, but it won’t push you towards anything.
Your goals need to be mostly fixed, and your design needs to be mostly flexible.
Some tips to achieve this: