Want to Be a Better Writer? - Make More Arguments Outside of Your Head

For all the writing attorneys do in service of explanation and persuasion, most of us don't consider how to hone and expand that ability. Below are several ways you can sharpen your pencil (timely metaphor in the information age), mainly by subjecting more product to the light of day:

1. Read (Or Listen To) Your Writing Out Loud. If it reads well, then it usually sounds pretty good, too. Hearing the way your "brilliant argument" comes across when actually spoken (after all, it is not so hard to win an argument in your head) will undoubtedly provide some insight. And there are tools that can read your briefs aloud to you. 
2. Accept Some Coaching. There are a great many accomplished writers out there, some probably a brief's throw from your office. But the ability to hear some constructive criticism requires the recognition that you've got room to improve. See above about the way we see the world from between our ears.
3. Write As Much as You Can. In addition to what you produce professionally, explore other ways to write. Consider keeping a journal, which will not only improve your writing, but help you Cage the Monkey Mind and bring you other benefits by (you guessed it) getting thoughts out of your head.
4. Publish (Share Your Writing). There are so many ways to play to an audience these days.  And as I mentioned previously, knowing that you have any audience (of one or of a thousand) makes you write more effectively (again, because you are creating a written argument to be considered by someone other than yourself). And don't let the idea that what you've created is not good enough for an audience. After all, perfect is the enemy of the good, and the benefits of gradual improvement and potential collaboration that come from sharing your writing far outweigh whatever flaws exist in your "perfect offering."

Of course, very little of the above is earth-shattering. But sharing what you create is the best way to make it better. As one fairly accomplished writer put it:

"Write with the door closed, edit with the door open." - Stephen King