Hmm... well, I'm not sure how to word it but...
First, make it brief. Get straight to the point lie if you were writing a persuasive essay. What is you story about? And then make sure you have impact. Open and end with impact.
My webcomic, Beautiful Lies, it's summary is this:
"It's a story about a group of adolescents living dull or miserable lives who seek comfort in their own fantasy world.
They all encounter a similar situation which transports them to a lawless, fantasy world that shapes itself to their inner desires. There is no time to age them, no rules to control them, no authority to subdue them.
As they search for a way to return to the real world, they begin to question if they want to leave at all."
Now, my opening is okay. It's direct but the ending has impact. Sounds like something you may want to peek at.
The idea is to have impact, at least for me. Now, that often differs depending on your genre. Shoujo and shounen have much different kinds of summaries than psychologicals but in general, they have impact.
I suggest to start with a premise of 15 words or less. What is your story? "A group of high school students are stranded on an island." Then extend it.