Read as much as you can get your hands on. Even if it's a book you don't particularly care for, try to see why the writing isn't holding your attention. Are there too many digressions or are the characters too thin? You can often learn more from a poorly executed piece of writing than a brilliant one. Take notes of how your favorite authors set a scene or build tension. Look at poets to see how they use line breaks and imagery.
Set aside time every day to write, even if for only 20 minutes, and give yourself goals, like writing 1,000 words or a complete sonnet in the session. The more you write, the better you'll get. Taking time off from writing makes it harder to jump back in. Your skill gets rusty, and you have to work harder to produce.
Give yourself timed writing exercises. For fiction writers, place two characters having a heated argument standing in line at a bank. Then, have a man enter the scene with a gun and tell everyone to get down on the ground. Have the characters react and possibly still continue their argument.
Poets and nonfiction writers can try to recall an event from their childhood. List as many details as you can remember in two minutes.
Cut clichés from your work. They make any piece of writing trite and stereotypical, so remove them completely or at least put a twist on them to make your work stand out. For example, look at Mark Twain's twist on the clichéd rule to count to a particular number when angry before speaking. Twain wrote, "When angry, count four; when very angry, swear."
Find joy in the process. When you read writers like Kurt Vonnegut or J.K. Rowling, you can feel the pleasure they received from stringing words together on the page. Even if it's an assignment for class, trying to focus on one particular aspect you enjoy and see the affect it has on the quality of your words. Thinking of creative writing as a chore will typically create the same effect for the reader. Read your work aloud to see where even you start to drift or trip. Show; don't simply tell, because too much exposition can kill any piece of writing. Experiment with alliteration to see if you can add a little fun to a boring sentence.