You've written this wonderful (no question about it) piece of work. It's the greatest thing since tealeaves were invented. The more you read it the more you admire your own genius.
Then the little doubt starts.
Now is the time to step back from the work itself, to close the book, the page, the computer file and think. Not of the words or the actions or of the characters themselves, but deeper, back into yourself.
# What made you write this piece in the first place?
# What were you trying to say?
# What point did you want to illuminate for everybody else out there in the world?
That sounds terribly grand and pretentious, but it's the truth. Every time you write, no matter how light-hearted the theme, you are giving away something of yourself, your values and your beliefs. And what you have to say is important.
# So what is the value of the piece?
# What is your theme?
Express your theme, the kernel of your thoughts, in one short sentence.
When you write you also have secondary goals. This is when you think of the piece itself as it sits on the page. The added value bit, the craftsmanship. The approach you took to get your message over.
Perhaps you used humour, perhaps the layout on the page.
What makes it interesting and fun / easy to read?
Remember, sometimes the best writing is what you leave out. Have you said the same thing in two different ways? What I call my double whammy.
And finally, don't forget the old adage. Pretend you're writing for an audience of fourteen year olds, and the adults might understand you as well.