Write every day, even if you feel dissatisfied with what you produce. The more time you spend writing, the more likely you are to see improvement.
Read a lot. Reading will expose you to various writing styles and techniques that can influence your own writing. Reading can also serve as inspiration to improve.
Study basic writing rules. Correct spelling, grammar and punctuation are critical in written correspondence. Know when to use words that sound the same but are spelled differently, such as "there" and "their" or "your" and "you're." Review the rules for using comma to avoid run-on or confusing sentences. Make sure you understand the correct context for using words that are often confused, such as "lie" and "lay."
Use specific language to bring your writing to life. Specific, concrete language creates visual images that make your writing more interesting and clear. For example, "The growling, 100-pound pitbull" paints a clearer picture for your reader than "The angry dog".
Use the active rather than the passive voice. "The ball broke the window" requires fewer words and reads better than "The window was broken by the ball".
Start with a clear main point and get to it quickly. Remove repetitive phrases and unnecessary words. For example, don't write "red in color" when you can simply say "red". Avoid words like actually, really and very; they often fail to add clarity. Reduce short phrases to one word, where possible. Write "now", instead of "at the present time."
Revise everything you write since no writing is perfect in the first draft. Take a break of at least a few hours between the first draft and the revision. This allows you to come back with a fresh eye.
Ask family, friends, classmates or teachers for feedback or join a writer's group for critique from others who are also serious about improving their craft.