Grammar - Depending on the language you use, blogging just makes it imperative that you are reasonably good with your grammar skills and proficiency if not a master. Remember your audience is a mix of both; the ones who are experts and the ones who are not.
Write regularly, neither too often nor too rarely. Two to three blogs a week is a good frequency to maintain.
Offer easy tips for any problem and your readers will return every time they need some advice.