(Examples, unless otherwise noted, borrowed from manuscripts I've critiqued in workshops.)
1. Show, Don't Tell. We all know what this means... or do we?
"Say that again," Jim said angrily.
Jim's eyes narrowed and his ears seemed almost to flatten against the sides of his head. "Say that again," he said.
See the difference? Of course you do. But why is the second one better?
The second is better because the reader gets only gets the facts and has to come to her own conclusion about what those facts mean. In the first, the writer is also acting as interpreter, in effect telling the reader "trust me, I'll draw the conclusions for you." It's inherently unsatisfying to have someone do the concluding for you. I believe there's a physiological explanation: what happens when you don't get a joke, and someone explains it to you? You understand it now, but you never laugh, right? For the emotion to catalyze, you have to connect the dots yourself. If you want your writing to have emotional impact (that is, to not be boring) you have to present the facts and let the reader draw the conclusions. Just like in a joke.
"Just be careful," Strunk warned. Do we need to be told this is a warning? If you need to emphasize that this is a warning, rather than, say, a loving recommendation, just write whatever it was that made you conclude it was a warning. Was it a tone in Strunk's voice? His expression? That's what the reader needs, not an interpretation. Just the facts, ma'am...
Counter example, on "show don't tell" and some other points, too:
He looked up as the handle on the wheelhouse door turned and Danny stepped through. The cabin temperature dropped ten degrees in the time it took to dog down the door behind him with a clang. Danny pulled off his chouk and shook the ice crystals out of his hair like Bill's Jack Russell after its bath. His cheeks -- what could be seen of them above his curly brown beard -- were bright red; each with a white patch in the middle the size of a quarter where the skin was just beginning to freeze.
A lesser writer might have just said, "Danny looked half-frozen." See the difference? Look again, and you'll see other things that are working, too, mostly in the details (the cabin temperature dropping, the white patches), and also in the imagery (a dog shaking itself, which might hint at something about Danny's personality? See how good writing accomplishes many things at once, as discussed below). Also note how unobtrusively the physical description of Danny is built into the scene.
2. Point of View (POV). Uncontrolled shifts in POV are always jarring. They can also tell rather than show. Examples:
"Flatt dropped to his knees, slid the computer out for working room, and pulled out a screwdriver."
In the descriptions of Flatt's various actions, the writer is way in the background and the reader is right there in the room, experiencing the action directly, without an intermediary. But in the middle of all that there's that "for working room," which feels to me like the writer's explanation for what's going on. It's jarring and inherently less interesting, too. Trust the reader to come to his own conclusions about why Flatt might have slid out the computer.
"High heels be damned, she ran down the street towards number Twenty-Eight."
The first clause is the character thinking. The second is the author narrating. You can't switch abruptly from one to the other without jarring the reader out of the story.
3. Detail. Everything you write has to be in the service of the story. If it doesn't serve the story, cut it.
So, how much detail should you give about setting, the characters' appearances, etc.? Remember what Abraham Lincoln said when asked how long a man's legs should be: "Long enough to reach the ground." Ask yourself: how does the detail serve the story? Why am I giving detail here?
4. Feed and Starve. All good writing conveys necessary information while simultaneously feeding a hunger for more information. Necessary information is always some variant on who, what, where, when, why, how. The best example I can think of is the opening of Ken Follett's "Key to Rebecca." Read it and ask yourself what necessary information Follett is providing -- that's what grounds you in the universe of the story. Then ask yourself what questions the provided information implicitly raises -- that's what hooks you. You need to do both: the first without the second is boring because it answers all questions and obviates suspense; the second without the former is irrelevant and therefore uninteresting.
5. The Five Senses. Engage all the senses -- not just visual. If you're describing a place, what are the ambient sounds? What does it smell like? Temperature? Humidity? How does it all make the character feel? How it makes the character feel ought to be relevant to some aspect of her personality, which in turn should advance the plot... see point 4...
6. Who and What. Nothing else will work if you don't first get the reader to care about your characters.
Sometimes people argue over what matters more in a story: characters or plot. The argument is silly because a story is both, plus setting (I think of these as who, what, and where). Character without plot is a resume. Plot without character is as boring as the recitation of someone else's dream. Setting without character and plot is a still life. You need all three, and all three need to work off each other.
But I will say this: humans are hardwired to care much more about who than they do about what. In December 2004, a quarter million people died in a tsunami. If you didn't know any of the victims personally, how much did it really affect you? How much was it on your mind on any given day? Conversely, if someone cuts you off in traffic and flips you the bird, how upsetting is that?
The first event is monumental, but you didn't care about who so you weren't affected by what. The second event is absurdly trivial, but it happened to the most important person in your world -- you -- so it mattered a lot. What does this tell you about character in a story?
If the reader doesn't care about who, he'll never care about what.
By the way, my definition of literary fiction is fiction that relies on who to advance the story. The more the story depends on what, the more you're talking about genre. If your characters are truly compelling, you don't need to have the fate of western civilization hanging in the balance to get readers to care (but it can't hurt, either). Of course, the best fiction has all three: who, what, where. Think "Lonesome Dove." Not a coincidence that it won the Pulitzer...
7. Keep Writing. Can't emphasize this one enough. Every day is ideal, but the goal is to just be as regular as you can. The process is similar to as learning a language, or a martial art, or a musical instrument. If you've got time and you're serious, a writer's workshop can be a huge help with motivation, feedback, and discipline. Google "Writer's workshops" and the name of your city and you'll probably be able to find a bunch
8. Read like a Writer. Reread passages from books you love and ask yourself, what is the author doing here that's working so well? And if you see something that you think is bad, ask yourself, why is this bad? What could the author have done differently to make it work?
Explain to yourself what techniques the writer is using. Because before you get to art, you have to master craft. (My definition of art and craft: it's a continuum, but generally speaking, art is what is unique to you, what would never have existed if you hadn't existed. Craft is technique. It can be learned, in fact can only be learned, through disciplined practice. Craft is all the things I'm talking about in this article. Using the five senses. Mastering control of POV. Highlighting the telling detail, the essence of a thing. Showing, not telling.)
9. Books on Writing. Stephen King's "On Writing" helped me a lot. David Morrell's "Lessons from a Lifetime of Writing" is also great. Sol Stein's "Stein on Writing" is packed with useful information. There are many others. But don't read the how-tos at the expense of your own writing. Whenever you have to choose, practice your writing instead. Novelist J.A. Konrath offers terrific writing tips on his website.
10. "What If?" One of your best friends as a writer is what I think of as the "what if" question. "What if someone cloned dinosaurs and planned to open a dinosaur theme park on a remote island?" ("Jurassic Park"). "What if a semi-yuppie drug dealer were about to do a seven year prison stretch?" ("The 25th Hour"). Etc. If the what-if question interests you enough, it'll lead you to other questions, all of the who, what, where, when, why, how variety. Follow those questions and you'll start to find your story.