Focus on grammar. A common obstacle to good writing is trouble with grammar. A lot of people struggle with creating sentences that make sense. As painful as grammar often seems, it's important for someone who wants to write well to work on improving his grammar. One way to do this is to take a basic grammar class at a community college. Some schools offer online classes for the sake of convenience.
Consider your audience. When you get ready to write something, the first thing you should do is figure out who your audience is. This will make a big difference in the type of language you use. For example, if you're writing an email to a friend, it's OK to use abbreviations like "LOL" or "KWIM." If you're writing an email to a business associate, however, you'll want to use language that is formal and correct--no slang or abbreviations. Knowing your audience will also help you decide on the level of your language. If you're writing an informational brochure for the general public, you'll want to use language people of all reading abilities can understand. If you're writing a report for a group of engineers, however, you can use specialized technical language.
Consider your purpose. Ask yourself, "Why am I writing this?" In general, writing can serve four purpose: to inform, to entertain, to persuade or to tell a story. Knowing your purpose can help you keep your writing on track. For example, if you're writing a letter to the editor of a newspaper with the purpose of taking a stand on an issue, you'll want to be sure your writing clearly states your stand and provides support for your ideas. If you fail to state your stand on the issue clearly or wander off your topic by telling a funny story or bringing up a bunch of other issues, readers will be confused, and your writing will be ineffective.
Use variety. Effective writing is clear and interesting. In order to accomplish these two things, try to vary your sentence structure, and use strong and creative language. If you have a whole paragraph of sentences that begin with "I am," as in "I am an experienced professional. I am good at managing people. I am organized and effiecient," your writing falls flat. Try switching things up a bit. The same goes for the words you use. Think about how much more impact you can create by saying, "The enormous, snarling dog" instead of "The big, barking dog." If you have trouble thinking of interesting words, try using a thesaurus.
Aim for clarity. Clear, concise writing is effective writing. To achieve clarity, be sure to write in complete sentences with correct punctuation, and use paragraph breaks. Ideally, a paragraph should express one main idea and several details that support the idea. When you are beginning a new idea, start a new paragraph. Have you ever tried to read something that was a page-long paragraph? It's overwhelming! Keep your paragraphs to a moderate length and focused on one idea at a time.
In addition to following the steps above, consider taking a composition class. Many community education programs offer classes to help people improve their writing. These classes often last only a few weeks and can be very effective. If you have the time, you'd likely benefit even more from taking a semester-long writing class at a local college.