Write with conviction. Pick a side and be bold. And if you're wrong, admit it.
Write regularly; not too often or too rarely. Two to three blogs a week is a good frequency to maintain. Depends on the genre though.
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.