*based on my personal experience as a slush pile reader. Please discuss in the comments if your experience is different.
Write Your Best
Analogy: You are heading out for a job interview. You know the mustard stain on your t-shirt probably doesn't match your suit jacket, but you're in a hurry, so whatevs.
Don't send stuff you're not happy with. Sometimes you've hit a deadline and you need to get something out, but try to minimise those times.
Sure, it's obvious, but obvious things often need to be said. Obvious things like...
This is so obvious you'd think it wouldn't need to be said: Read the Friendly Guidelines.
Analogy: you want to borrow some money from a stranger, so you punch them in the face first.
This is the number one reason why people get rejected and if you didn't read the publisher's guidelines you obviously don't care about their needs. Guidelines exist to make it easy for editors/publishers to organise the massive flood of often mediocre unsolicited work through which they have to dig. Do not obstruct their processes unless you don't care to be published by them.
Note that some editors will be more forgiving of this. But ask yourself, 'Am I the kind of person who likes to play Russian roulette with my career?'
Value Basic Technical Quality
Analogy: You want to build a tree house, so you buy balsa wood and tacks, and use the back of your screwdriver to bang it all together.
This means pay attention to spelling, basic layout, and grammar. It's the kind of technical stuff you need to learn before you can do anything, so get on it. Practice and exposure (through reading and editing) makes perfect. It's really jarring to read stuff that even the author hasn't bothered to proofread.
Analogy: You arrive at your graphic design job interview with a brass band.
If you don't try and experiment you're going to be dull. Since pretty much everything you can think of has already been done, THINK HARDER. (Sorry, I can't be more helpful than this). Just like in the programming world, ideas are cheap and implementation is everything. If you put your own characters and voice to an idea, then even if the core idea isn't original, your take on it will be. Don't play it safe.
Analogy: "Syphilis! Now that I have your attention..."
Consider that you have about three paragraphs (I'm being generous) to capture the editor's attention, so don't mess around at the start with exposition or stuff that doesn't represent the rest of the story. Spending a few pages faking out the reader by pretending your SF horror story is actually Noir means that the editor will put it in the Noir pile and may not get back to it if they fill their quota with other people's SF horror stories.
Unfair, isn't it? Someone reading for an anthology with a certain theme is going to expect your story to turn to that theme eventually, and so that's the kind of market for which you can try a story with an approach like that. For a magazine, though, it's quite different. If a given edition needs more romance stories, then your romance story that starts out looking like a horror story might be overlooked by accident, a victim to efficiency.
Analogy: Wear a large, colourful hat in an unusual place, such as the Department of Motor Vehicles or a funeral.
This occurred to me as I was looking through the extensive pile of last-round stories submitted to a magazine I slush-read for. I placed myself in the shoes of an editor who had to pick ten stories out of about a hundred that had passed the initial slushing round and the secondary approval round, and were now considered 'good enough to be in the magazine'. Consider that the editor hasn't seen or read any of the stories yet: the stories have gotten this far through sub-editors and volunteer readers.
Glancing over them I initially filtered by their score - basically that cut the list down by about a third or so, leaving 60 odd stories. Then I skimmed over the titles to determine what I might find interesting. I had to, since there was no other way to decide where to start: I could either begin reading at the top of the list of 60, or begin with the most interesting titles.
Obviously editors will skim author names too, so if you're recognisable or well-known you are also more likely to get eyes on it early (which doesn't mean a thing if the story's no good, btw).
On a side note, skimming for author names will also work against you if you've been a jerk online. Word gets around. Don't Be That Jerk. I discussed this yesterday.
I hope those points help a little. Tomorrow I will return to my regular schedule of digging up articles I find interesting and commenting on them.