Now I have yet to achieve the sort of success that prompts this question, but I have friends who have herd it many times and it is the cliché for something creative people get tired of hearing. However I have thought a lot about this question. Not only from a perspective of that someday I hope to have the success that prompts it but also thinking about why it is asked in the first place.
First of all I do not doubt the earnestness of those asking this oft repeated question. I think that they want to be creative people and looking at someone successful who has produced plentiful ideas and that their own fields seem so fallow it is natural to wonder if there is some process of trick that turns a person creative. It is not those they that there is a single source of ideas, jokes about a PO Box aside, they understand that creativity is process. It is a process that looks mysterious and I believe that they want a little help in getting that process started.
The sad truth is that there is no answer to the question. For each and every creative person there are multitudes of paths to a workable concept. For me there is a commonality to my paths and that is most often my idea start as questions.
Watching an episode of Star Trek (The original series) where they have found yet another duplicate Earth I asked myself what might actually produce a doppelganger of our planet? (Aside from limited budget on your production.) Answering that question became my most recent sale, A Canvas Dark and Deep. I have idea sparked by doing the dishes when a floating lid looked like a strange watercraft and I started asking questions about who would build that and why. What if humanity moved out to the stars but not unified but still yoked to nationalism? That became a series of novels. If there are ghosts why are they so rare when there are so many people? That has spawned a couple stories as I have explored different answers.
If you want to be creative the only suggestion I can give you is ask lots of questions. Particularly you must question that things that every just assumes. Flip things and ask what is the case if the opposite is true instead of what every thinks.