With my current manager this has become a skill I'm honing. He's senile and asks for all sorts of things he will have forgotten about in a few days. So I slow roll any new idea he gets excited about until it has percolated for at least a week or two. The trick is figuring out how to wait long enough to avoid doing throw-away work without waiting long enough that he feels ignored. But, like I said, he's senile, so that helps with the latter.