Avoid using re where concerning, regarding, or about will do the trick, as in “Re your memo of 13 January . . . .” It makes your writing jargony.
When you're faced with a stylistic problem you can't easily solve, it's often wise to scrap the troublesome sentence and start from scratch, perhaps using a completely different construction. For instance, if you're bothered by a problem with his and her — “Just as a musician has to be a master of his or her instrument, a writer is at his or her best when he or she has mastered his or her linguistic tools” is downright cumbrous — scrap it all, and use something like “Mastery of words is as important to a writer as mastery of an instrument is to a musician.” There's nothing wrong with avoiding such problems; he who fights and runs away lives to fight another &c.
Pay attention to redundant words and phrases, as in actual reality and anticipate for the future. See Different.
The writing process isn't over when you reach the end — it's hardly begun. Pay attention to a maxim often quoted in composition classes: “There is no writing, only rewriting.” And that means much more than simple proofreading. You should always spend a lot of time revising your work — looking not only for outright grammatical errors, but also hunting down wasted words, improving clarity and precision, and working on your transitions. I know it pains beginning writers to hear that they have so much work to do, but it's really unavoidable. Not even professional writers get it all right in the first draft, so you should be prepared to put as much energy into revision as you do into the original composition.
A tip: let some time pass between your first draft and your revision. That means you have to start putting things on paper well before the deadline. Don't worry: it needn't be perfect; just write. And then forget about it — for as long as you can afford. (The Roman poet Horace suggested taking a nine-year break between composition and publication, but few of us have that luxury.) When you come back to it, you'll be able to read your own writing with fresh eyes, and to see things you missed before. It makes all the difference in the world, but you have to start well before your deadline.
Mind you, it's bad faith for me to pontificate on the moral turpitude of the procrastinator: I'm one of the worst offenders. Do as I say, not as &c. [Revised 3 November 2000.]
Are rhetorical questions ever useful? Yes, they are. Are they, however, grievously overused? Yes indeed. Do they contribute anything to conversation? Almost never. And do I think they're worst when politicians and other dimwits ask themselves what they think? Without a doubt. Do I think the level of public discourse would be raised if the aforementioned dimwits would lose this annoying verbal tic? Absolutely. [Entry added 14 Sept. 2004.]
There ain't a rule in the language what can't be broke. The so-called rules of English grammar and style were not spoken by a burning bush; they're just guidelines about what's likely to be effective. If you learn to treat them that way, you'll live a happier life. To that end, read my entry on Prescriptive versus Descriptive Grammars.
Here's how to think about your task in writing. Step one: figure out where your audience is now. Step two: figure out where you want your audience to end up. Step three: take them from one to the other. If you can pull it off, anything goes.
Does that mean all the entries in this guide are superfluous? Not at all. The question is how you can drag your audience from one to the other — an audience filled with people of widely varied knowledge, backgrounds, and prejudices. You don't get to pick what hangups your readers suffer from: you have to take them as they are. Writing is an inescapably psychological game, since you have to crawl inside your readers' heads and figure out what's likely to have the desired effects on them.
That's where the rules come in: they're attempts to lay out systematically the effect certain usages will have on certain audiences. A rule that says “Don't split an infinitive” can be translated, “If you split an infinitive, then at least part of your audience will think less of you, and you're less likely to win them over.” If you break these rules without a good reason — by which I mean a reason evident to your audience — you lose your audience. It's that simple.
Rules are tools. Don't think of them as bureaucratic regulations designed to get in your way, and don't think of the chance to bend them as a special treat. Instead, think of them as a collection of techniques that are likely to have the desired effect on your readers.
A corollary: there's no single set of rules. Every style, every genre, has its own guidelines. A Nobel Prize speech demands a different style than an MTV Music Awards speech (to my knowledge, the former has never included the word “dudes,” nor the latter the word “ineluctable”). Most of the guidelines I lay down here are appropriate for college English papers, a genre calling for a middling degree of formality — that's also roughly the level that most business communication should have. Other styles have other rules, and all you can do is learn what works in what genre. Keep your audience constantly in mind, and learn to use the rules — even the ones you find silly — to win them over.
The one unbreakable rule: Whatever works works. All that's left for you is to figure out what works. Most of us will spend our lifetimes on that puzzle, and the so-called rules are the closest thing we have to a solution.
Just as there's nothing inherently wrong with a long word, there's nothing wrong with a long sentence. But it has to be grammatical. A run-on sentence is ungrammatical, not just long. It often happens when two sentences are run into one without the proper subordination or punctuation. Two sentences glued together with only a comma produce a comma splice, a kind of run-on: for instance, “The semester runs through April, the break begins in May.” There are a number of ways of fixing this comma-splice: “The semester runs through April. The break begins in May”; “The semester runs through April, and the break begins in May”; “The semester runs through April; the break begins in May”; “The semester runs through April, whereas the break begins in May,” and so on. See Semicolon and Dependent versus Independent Clauses.