When you think you have found the big, beautiful, shiny idea that your essay will focus on, try to whittle it down. And don't speed through the process—take a whittle time.
Ask yourself "why" and "how" over and over again like an obnoxious, sugar-addled three-year-old. You can even try spilling grape juice down the front of your shirt if you think it will help. This persistent questioning will allow you to find a topic that is specific rather than having a vague, fuzzy idea. Even if the fuzzy ideas are better suited for cuddling.
Example: "Growing Pains is the best TV show ever produced." Why? "Because it taught me about the true value of family." How? "Through the tumultuous but loving relationships between Mike, Ben, and the Dad." Now you have a focused idea for your essay: "Growing Pains is the best show ever produced because the tumultuous but loving relationships among Mike, Ben, and Dr. Seaver teach the viewer about the value of family." It's a mouthful (and a pretty crazy-sounding mouthful), but at least it's a focused one. And it's totally true. Growing Pains is awesome.