Learning to write well is a continual process. When readers ask me about how to become a better writer, I always say, "First, write every day." And I mean it. With writing, the only way to get better at it is to keep at it. Writing doesn't improve just by thinking about it.
Then I give this advice: "When you write, don't worry about misspellings and grammatical errors. That's what editors are for." Writing and editing are completely different skills. Great writers often make spelling and grammar errors, and great editors and/or proofreaders are often better at catching mistakes than creating compelling stories.
When writing for our jobs, however, we must often be both writer and editor extraordinaire. Also, if you are a blogger and don't have anyone else to review your work before you post, then occasionally reviewing lists like these from TechRepublic Senior Editor Jody Gilbert can help you catch your own errors:
Here are two simple, additional tips I recommend for self-editing your writing:
1. Read your work aloud. The ear often catches errors that the eye does not.
2. Do spell check. (Seriously. While it doesn't fix words used out of context, it does catch misspellings. Use it!)