I have volunteered as an expert on business, technical, and academic writing at AllExperts.com for about three years. In that time, many have asked how best to improve their writing skills or how to help their children write well.
I usually answer either question this way. Writing is a learned skill just like hitting a baseball, throwing a football, or hitting a golf ball. It is not a "gift." If there is a gift in writing well, that gift is love and respect of the language. The best way to acquire that love is to read. So if you would write better, read. And read good stuff, not just the daily newspaper, however valuable that might be.
Read things that will expose you both to good writing and to knowledge that will be worth having. After all, you will need something to write about. Read good novels, histories, and biographies of people whose lives are worth knowing about. Read about science and some of the ways great scientific discoveries have been made in the past. If you are a parent of young children, read to them; again, read good stuff, not just the toy readers that you can buy in children's stores. Read classic kid's stories. And let your children see you reading and enjoying it so they understand that it is fun.
The other thing to do is to practice writing. It's a learned skill, remember. About 30 years ago, I took a seminar on proposal writing. The person who taught it advised us all to write at least one page every day, even if it was only a letter to our grandparents. And when you write, he said, look at what you have written to make sure you said what you meant to say. Today, I advise everyone to take care with everything they write, even casual e-mail. It takes very little time to do it well, and usually saves time in the long run. And each memo or e-mail is an opportunity to practice writing--and by extension, thinking--well.
If you do these things, you will become an effective writer.