Shai,
it took me a month to come up with my "hook," but once I did, the statement came together rather quickly.
I took a moment out of my life that was personally and academically significant and built upon that moment. For example, "I stood among the hills of (place where significant thing happened)... (writing, documenting, observing, etc.)... I found myself contemplating..." and then tied it into the research of the professors/universities I was applying to, paradigm shifts in my field of interest, and the foundational theories that shaped my undergrad and grad work.
I also used a professional editor after my essay was complete. I'm a writer, but the editor saw things that I had missed. He had also seen so many essays that he knew what "worked" and what was a bit trite or flowery or "not quite right." The editor had a few suggestions to tighten the opening statements/paragraph.
Hope that helps.