Dan Meyer has written an excellent post on how math educators can blog and tweet (and there are over thirty quality comments there now, too), and I think it holds true for all edubloggers.
Here are a few snippets of what he says he tries to do as a blogger (though I would strongly encourage you to go over to his post and read it all), and it sounds good to me:
I try to turn interesting opportunities into blog posts.
I answer every email.
I try to highlight the best of new blogs.
I try to create more value here than I’ve captured.
I’m adding this info to The Best Sources Of Advice For Teachers (And Others!) On How To Be Better Bloggers.