Read everyday. Your reading level will increase if you commit yourself to cracking open a book and spend time poring over the words, sentences and punctuation of others. Paying attention to word choice, sentence structure, and the flow of words within a particular sentence will increase your familiarity with the way things are supposed to read and sound. When you go back and re-read your own work to improve it, you'll have a far more developed sense of what works and what doesn't from your extensive knowledge of solid writing.
Memorize grammar, spelling and punctuation rules. To become a writer capable of being clear and accurate, it helps to have a solid understanding of grammar, spelling and punctuation. Certain rules such as "I before E except after C," or "sounding like A in neighbor and weigh," knowing when to use who/whom and the right place to insert a comma can make the difference between communicating your ideas and not being understood.
Read your writing aloud or, better yet, have someone do it while you listen. This is an effective way to check if what you've written is clear and makes sense. Awkward wording, misspelled words, missing words and punctuation will become obvious when the words are heard instead of read. After hearing it aloud, you may find there is a more efficient to relay your message, or perhaps you need to elaborate further to get your point across.