Write a nucleus word or phrase on a clean piece of paper. I usually choose a word that I consider, loosely, to be my topic. For example, if I'm writing a Mother's Day essay, "mother" would be a good nucleus word. If I'm writing an annual report for a client I might choose "service" or "business" or even a phrase like “improving our image." The nucleus word's purpose is to trigger associations. Emotionally charged words like "love," "loss," or "envy" are extremely effective, as are prepositions: "around," "beyond," "over," and so forth.
If you're stuck in a writing project, cluster.
Don't think about it at all, just keep clustering words until you find your inspiration again. You will -- this technique always works.