3. Forget about being pretty.
Agents and editors can't stand authors who put restraints on their work for the sake of delicacy.
A few years ago I was teaching a workshop and trying to get across the concept of writing freely (with no thought of whether you like the result).
A participant spoke up: "I once had an art instructor say, 'If it didn't have to be pretty, what would you draw?' "
I practically reeled from the force of the genius of that question. (Thank you, anonymous writer and unknown art instructor!) Everyone in the room immediately made the translation: "If it didn't have to be pretty, what would you write?"
Here's the key: Not-pretty has two meanings here: a) topics that are not attractive, like racism or incest, and b) the way you write.
Most people shy away from darkness, but as an author you must be willing to dwell there, see it truly, explore it before you represent it.
I kind of hate to say this, but I advise going back to your childhood years—the primal times before we really knew right from wrong, and before we were strong enough to defend ourselves from evil. Feel the fear that coursed through your body when you saw the neighborhood bully coming. Feel the shameless intoxication of wrecking something out of spite.
As for freeing up your writing, do the same thing. When you were a kid, you did everything with almost complete abandon. Call up that spirit as you put pen to paper or fingers to keyboard. Banish all restraint!
4. Be true to your IQ.
When I worked for a large bookseller, we ran surveys that showed our core customers to be well educated and fairly affluent. This was not surprising: Educated people tend to like books, and their income tends to enable them to buy books.
Still, aspiring authors sometimes dumb down their work because they're afraid of alienating the vast masses of potential customers they imagine they should be writing for. This is disastrous. You cannot do it. And you don't need to—the average Joes and Janes are smarter than you may think.
Here's the key: Don't underestimate your readers. If they like to read the sorts of books you like to write, they're right up there with your core demographic. And dumbing down your work can be doubly disastrous, because if you do, agents and editors will not be able to relate to it.
First, free your vocabulary while also keeping it in check. If abhorrent is the right word, don't change it to yucky. And when hill is the right word, don't change it to acclivity just to show off.
Second, resist the urge to overexplain, especially when portraying action sequences and characters' thoughts.
Edwina stopped revving the accelerator. The car rocked back into the sand. She looked up at the thick spruce boughs that hung into the road. She got out and said, "Help me pull some of these down."
We do not need to be told what went through Edwina's mind; we can conjecture just fine.
Agents and editors will recognize an honest, unstilted voice, and they will respond to it. As will your future readers.
5. Use your best material only when it has a purpose.
Agents and editors have a sixth sense when it comes to kitchen-sink novels. You know what I'm talking about: novels that contain a fictionalized version of every cool, unusual or amazing thing that ever happened to the author.
I once read a novel manuscript at the insistence of a friend who knew the author. In it, a man on foot stops to talk to a man on horseback who is wearing a live snake around his waist like a belt. The incident was colorful but had no bearing on the story, and I suspected that the only reason it was there was that the author had once met up with a man on horseback who wore a snake around his waist like a belt. A casual inquiry proved me right.
An isolated cool-yet-irrelevant scene suggests the author's immaturity as an artist, and will be noted by agents and editors.
Here's the key: Put your best material in, but leave the kitchen sink in the kitchen.
When tempted to throw in something awesome that the story doesn't really demand, go ahead and write it, but during revisions take it out and save it.
Alternatively, adapt your story to the cool thing. The author with the snake-belt guy might have brought that character into the story more, either by making him a one-shot oracle who gives or withholds a crucial piece of information, or by making a real character out of him, with a name and a crime or a heartache. The snake could then have served multiple purposes: to show the character's determination to be different in the face of social convention, for instance. Or maybe he just doesn't understand why he can't get a girlfriend.