Step one: let go of your fear. Putting your thoughts on digital paper and publishing for the world to critique (for better or worse) can be intimidating even to experienced writers. The good news is that you don't have to be Shakespeare, or even J. K. Rowling, to write a successful post. If you can hold a conversation, you can write a decent blog. Your knowledge matters more than your grammar skills.
Start by writing a short, catchy title. Numbers often work well for grabbing attention (for example, 5 Tips for Blog Writing), as do questions. Remember that titles are also useful from an SEO perspective, so try to include a keyword or two.
Be yourself! Use your own voice, write conversationally, and let your personality shine through. Don't get caught up in corporate-speak, jargon, or buzzwords. If you have to use unfamiliar or technical terms, link out to a plain-English definition or wiki.
Keep it short. The ideal blog post is between 300-500 words. More technical or complex ideas may stretch to a maximum of 800 words. Anything longer than that and you risk losing your readers' interest.
Open your post with a brief intro that states a challenge, asks a question, or makes an otherwise intriguing point (such as a statistic).
Whenever possible, include an image -- whether that's a chart, a photograph, or a screen shot. Blog posts with visuals perform better.
Use bold text for emphasis.