Do you have something to say? You should, but do you, really? Writing for the sake of writing is good practice, but writing for the sake of communicating is good practice and good purpose. Have something to say and say it.
Read what others write. Freelance writer Melanie Brooks correctly states, "If you don’t like to read, you can’t possibly love to write." You learn about writing, often subconciously, by reading. Read good material. Read a variety of styles and types of writing. Those who read my "books to buy" blog piece yesterday realize that my reading is fairly constricted -- mostly history and theology. I almost never read fiction any more. But I am trying to branch out a bit. I recently bought An Atheist Defends Religion, a little off the beaten path of most of my reading. Two days ago I picked up a copy of Hemingway's The Old Man and the Sea and started reading.
Read the King James Bible. Reading the Bible first and foremost is a spiritual exercise. The King James Bible is also a magnificient piece of literature and foundational to much of our speech, thought and thought patterns. It connects English speakers across time and space. Sayings and ways we say things are found therein. You need to connect with it.
Read rules about writing. Learning rules alone will not make you a good writer. Rules are not sacroscant, but they are compiled for a reason -- to pass along what someone has learned about communicating well. Write your own “Rules for Writing.” You may have learned some things about writing on your own. Let others review and critique your rules. Perhaps writing your rules can help your writing rule. The goal of writing is not to follow rules. The goal of writing is to communicate. You may want to communicate to teach, entertain, motivate or have a variety of other reasons. Whether you follow the rules or make your own rules, do you communicate? Do you get what's in your mind into the minds of others?
Practice, practice, practice. (In other words, WRITE.) Practice may not make perfect, but it does make better. The more you write, correct and edit what you write, the better your writing will become.