Writing is not really an art. It is a craft, and crafts can be learned. Here are four techniques you can use to immediately improve the quality of any writing you do, from business letters to web site articles:
1. After you run your spell checking software, go back and reread your writing. Very few people are good spellers, which is why spell checking software became popular years ago. Unfortunately, reliance on the software leads to silly errors. For example, the words "form" and "from" are both good, legitimate words. But if you wrote a business letter that said, "We will be taking 200 form your checking account to cover the payment," would your spell check software catch the error? Yet a simple typo changed "from your checking account" to the silly "form your checking account. Spell checkers are a convenience, but they are neither authoritative nor infallible. Always read through your writing at least once after you spell check -- and keep a dictionary handy.
2. Use that famous "KISS" principle for your punctuation. You know the rule, don't you? "Keep It Simple, Stupid" is the rule regarding punctuation. If you don't know how or when to use a semicolon, then avoid semicolons; you'll only display ignorance if you get it wrong. In keeping with that KISS principle, limit your use of commas. Far too many people use far too many commas. Just because a sentence is long does not mean it needs a comma. Well-placed commas make reading easier and are appropriate. Commas thrown in simply to break up the words are incorrect and distracting. Another important rule of punctuation is to avoid using exclamation marks almost always. If your choice of words, sentence structure, and overall prose don't convey the sense of excitement you are seeking, an exclamation mark won't do it. If your writing conveys your sense of excitement, an exclamation point is, well, pointless.
3. Make sure your writing is grammatically correct. You don't have to be an English teacher to get the grammar correct. You simply need to learn the basics -- verbs and subjects agree in number, for example. That is, "he was" is correct; "they were" is correct. To say "they was" is incorrect. If you aren't sure about using grammar, especially if English isn't your "first" language, go to some good reference sites for fundamental English grammar and usage. (If you are writing in another language, the same advice holds true for that language.) Invest in a good, basic grammar book or style manual. Check with any bookstore, online or offline, and you can find one.
4. When you finish some writing, put it aside for a couple of hours or days (if possible), then reread it before you let go of it. Of course, if you're writing or dictating a business letter or other "time sensitive" document, this may not be possible. If you set your article or story aside for even a day, then reread it, you may be surprised at glaring errors or significant changes you will want to make. And you thought before that it was finished. Many writers and teachers have commented that all true writing is done in the rewriting. Take that advice to heart and you will significantly improve your writing.
These four steps, rechecking your spelling, taking care with punctuation, watching for fundamental grammar errors, and rewriting, will improve your writing. They are proven tools used by every word craftsman.