I once had a creative writing student with a marketable manuscript who was absolutely convinced that no one would ever look at her work. She had no writing experience, no publishing history, and no connections. But she did have a PhD in Microbiology, a penchant for pigs in tights, and an adoring audience of preschoolers. Now I don’t know about you, but that combination alone would pique my curiosity.
And that’s what you want in a query letter.
It’s a one-shot, one-page opportunity to grab an editor’s attention and convince them that you, and your work, are worth looking at. Given the huge numbers of writers out there these days, you’re competing for a very small piece of the pie.
So make sure you are directing your query, be it email or online submission, to the appropriate editor; that the magazine and/or publishing house 
is the right one for you, and that your market information is up-to-date.
Pretty basic stuff, but after a year – or two – writing, editing and polishing your manuscript, even a simple mistake can lower the odds. Nobody knows your story as well as you do, but you have to be able to describe it in one or two paragraphs and hit all the right notes: concept, conflict and characters.
And should you have already met the editor you’re approaching at a conference or a workshop, do mention it. It never hurts to have a connection no matter how small it is. Failing which, find an angle which makes you and what you have to offer unique.
Be brief, be professional, include all your contact information and other details, like word count and series potential, list any publishing credits you may have, then close with a hook — and wait.
If you haven’t heard back after a reasonable amount of time (opinions vary on what constitutes a “reasonable amount of time” from six weeks to six months), find a way to remind the editor who you are and what your project is.
Case in point: I had written a query letter to a major publisher and received a request to submit a synopsis and the first three chapters of my novel. Which I did. I was also keen to establish a relationship and keep the process moving so I followed up with an email about six weeks later.
Here’s my opening paragraph:
“While I realize it’s a bit soon for a response to the first three chapters of Remember This, I thought I should let you know (were you to request the complete manuscript) that I’ll be in the U.K. until early April, visiting family and walking in the footsteps of my latest main character, thirty-year-old silver expert Gillian Maxwell.”
I went on to “pitch” my new story in the second and third paragraphs with a reference back to the book currently under review. I didn’t sell that original submission, but this second letter prompted another reply and another request.
And as soon as I finish that manuscript, I’ll be on to the next.
Good luck with yours!