Julie Andrews said it best in the song, Do Re Mi, "Let's start at the very beginning, a very good place to start... "
Once upon a time is certainly an acceptable beginning. Let's see if we can go beyond the utilitarian, though, to something that will grab your audience and make them want to know more. Here are some examples:
There was only one thing that could keep Cassandra from going out that night.
We want to know what that one thing is.
The world is a dangerous place, and wooing a young girl is just as perilous.
We want to hear about the danger.
Sampson was a great warrior, but today was not a day for victory.
We want to know what went wrong.
Ezmeralda was truly beautiful, but she was cursed from the day that she was born.
We want to hear more about the curse.
Let the audience know a little about the story, just enough to tease them. Make them curious about the characters, or about what will happen. The thunder and lightning frightened Amelia, and she knew that danger would visit her before the night was over. Don't we all want to know more about what happened? What was the danger, and even more important, "How will she survive?"
What’s the best thing that could happen to your protagonist? Tell us what that would be, then tell us that it didn’t happen that way in the story. Then...tell us why in the story.
Or you might tease the audience with, "The end of this story will surprise you. But in order to be surprised, we must start at the beginning."
Remember, your goal is to set the scene, and make the audience want to hear more!