Most GMAT students I've met really don't worry all that much about the Analytical Writing (AWA) portion of the test, but I occasionally meet somebody who is absolutely neurotic about this part of the GMAT. In my opinion, there's rarely any need to spend more than a token amount of time on AWA. First of all, one of the main reasons why MBA programs care about the GMAT is because of rankings. Unfortunately, US News & World Reports will look less favorably on a b-school if the program has low average GMAT score, so admissions committees are forced to obsess over your GMAT composite results. (Of course, there are plenty of other reasons for admissions committees to care about your GMAT score, but that's a story for another day.)
Of course, the AWA section has absolutely nothing to do with your composite score, and it therefore has no impact on MBA rankings. You don't want to completely screw it up, but there's no reason to believe that your AWA score of 5.0 would keep you out of Stanford or HBS. The adcom might start to wonder if your AWA score is, say, 2.5, but I see no reason to worry if your score is reasonably high. They're looking for business leaders with decent communication skills, not novelists.
So what does it take to earn a "reasonably high" score? Not much, really. I've seen some really, really bad writers earn scores of 5.0 or 5.5. They took advantage of the fact that nobody really reads your GMAT AWA essay.
As you probably know, GMAT essays are graded by a computer and by a human; if there's a discrepancy between the two scores, then an additional human reads the essay. Obviously, the computer's exact scoring system is a closely guarded secret, but I'm convinced that the program primarily looks for structure using basic keyword algorithms. From there, it probably counts things like the average number of words per sentence and the average number of letters per word, and then it checks for grammar, usage, variety of words used, and perhaps spelling. I suspect very strongly that structure is the most important part of the AWA scoring system; spelling might not even count at all. (Again, I've seen some horrid spellers get really good scores.)
The "human," for his part, is ultimately looking for the same things as the computer when he grades the GMAT AWA: structure, clarity, correctness, richness of language, etc. But this human probably doesn't actually read your GMAT essays any more closely than the computer.
Again, imagine that you're the poor slob who gets paid $20/hour (as of a year or two ago, GRE and TOEFL graders earned $20/hour; I don't know whether the rate is similar for GMAT) to read bazillions of these AWA essays, most of which are tragically bereft of interesting content. Are you going to read every single word? Hell, no. You'll skim the introduction, maybe read the first sentence of each paragraph, and then maybe skim the conclusion. (And if anybody asks, you'll tell her that you read every single word with great care.)
When I wrote about the people who read (or skim) your MBA essays, I claimed that you should strive to make your essay interesting--you want them to read it carefully, right? For the AWA, you don't necessarily care if the human grader (or the GMAT computer) actually reads your essay. You just want to earn a decent score, with a minimum amount of exertion. If you can conserve energy during the GMAT AWA section, you'll be much sharper when you battle the (far more important) quant and verbal sections.
So focus on structure on the AWA, even if that means that you produce an essay that would, under normal circumstances, seem like a crappy, contrived piece of writing. Don't beat around the bush: every paragraph should start by clearly stating the main point of the paragraph. The entire AWA essay should start with a crystal-clear thesis statement. Somebody should be able to read the first sentences of each paragraph, and still basically understand the entire essay. Just make sure that you leave a few minutes to proofread your AWA essay, simply to avoid committing any egregious grammar, usage, and spelling errors. (If this means that you don't write a terribly long essay, that's fine--quality matters more than quantity.)
Honestly, it's almost that simple. If you lay out your argument in a completely clear, five-paragraph format, you'll be in good shape, especially if your writing mechanics (grammar, usage, etc.) are basically solid. You might ultimately write an essay that is about as much fun to read as a phone book... but fun isn't really the point of the GMAT, is it?