I am quite fond of the YAGNI principle because it helps me concentrate on the essentials of the application currently under development. Another explanation is that I am getting lazier with age.
YAGNI tends to sell well with developers. It prunes needless work. However, with customers who ask for features, the YAGNI principle does not sit quite as well. People in general do not appreciate their decisions to be questioned and YAGNI can be resumed to one question. "Do you really need this feature?" The answer is often yes, forcing the skeptic in me to repeat the question in perhaps a modified form. Most people, customers included, do not like to be challenged, especially if done with some insistence.
Pruning requirements to mere essentials takes both work and courage. In the eyes of the customer, the alternative, i.e. asking for potentially useless features, may often look both easier and less risky.
I try to use the arguments as advocated in the c2 wiki, the feature that is implemented by anticipation now may be radically different than the feature actually needed in the future.
So how do you apply the YAGNI principle in a real-world environment? What are the arguments that may sway your customers or fellow developers?