I have learned this time and time again.
Yes, you have brain stormed on the new features for a long time, and yes it is an elaborately well thought-out plan. But please,
Be your own customer.
Start by writing the User Manual for the feature that you envision before the development begins. If you are going to release the feature over multiple iterations, write the manual for all the customer-visible iterations. This helps you shed the developer mindset, and forces you to really be in the customer's shoes every step after step. Obviously some activities are exploratory in nature, and don't quite fit in this format. But most of them usually do.
Easier said than done... :)