To get anywhere in the job market today, there's no getting around the need to be an effective writer. Your resume, cover letter, thank-you follow ups, presentations, and even your daily e-mail, are judged on how skillfully you arrange and rearrange 26 letters into persuasive and lively extensions of your personality. Like any other skill, doing it well takes practice. Here are some things to think about.
Become Aware of Good Writing
There's a nifty trick the Vipassana Buddhists have in teaching awareness. They simply ask you to notice that you're seeing. And what do you know - it works. Do the same thing when reading. Seek out good literature. Pick up a well written business book. Be aware of how authors use words to make strong points. Watch for effective techniques from good authors, and then adapt them for yourself.
Refine Your Work
I know you can't create three successive finely edited drafts of each e-mail you send, but when you can start early on written work, and make changes to your memos and presentations based on what you learned by reading the work of others.
And make sure you never employ a form of the word, "loose," when the intended meaning is a form of the word, "lose." That's the kind of error that can quickly get you tagged as a loser (not a "looser").
Study the Basics
Smashing Magazine has a nice little list of writing helpers. Of course, being Smashing Magazine, there's no top 10 or even 20. There's 50. So find what works for you, and keep going back to refine your communications even more.