Describe the world through the eyes of your characters. Modern fiction is rarely written from a truly omniscient point of view. Whether in first person or third, a story reflects the perspective of its characters, usually one in particular. Explore how a character looks at things and then write your descriptions accordingly. For example, a poet and an architect will see the same room in very different ways. Description and characterization should work hand in hand.
Use description and setting to reinforce the themes of the story. A description of dark woods could greatly enhance a story about a character discovering the dark side of his personality. But don't try to determine a story's theme too early in the writing process. Often your setting and the physical objects you choose for your story will give you clues as to what the story is about. Trust the physical world of your story and describe it faithfully. When you have discovered what your story is really about, go back and make sure all of your descriptions are consistent with the story's theme.
Use active rather than passive verbs in your descriptions. "Leaves blanketed the lawn" is a stronger phrase than "The lawn was covered with leaves." Go through your first draft and circle passive verbs like "is, was, were and are." Avoid passive constructions when describing a character's actions as well. In particular, avoid sentences that begin with a present participle (the -ing form of a verb). "Leaving for work in a rush, he forgot to feed the dog" should be rewritten as "He left for work in a rush and forgot to feed the dog."
Use details sparingly and with purpose. Instead of writing an entire paragraph to describe a certain room, find the one thing about the room that sets it apart from any other. In fiction workshops this is often called the "telling" detail. First drafts will often have more detail than is necessary as the writer works to discover which details are essential and which are not. Don't use descriptive passages as a way to show off your writing style or vocabulary.
Read and study the work of writers you admire. You can't become a good writer without being a good reader. Keep in mind that writers read differently. Read a story all the way through just to enjoy it, then go back and study how individual details contribute toward the overall effect of the story. If you find a particular passage that you like, copy it out by hand. Doing so will help teach you about syntax, sentence structure and rhythm.