I think that the biggest thing is to know what you are doing.
People are going to find and read your blog. They will search for and monitor particular words and phrases and see what you are writing on that subject. So if you produce something lame, people will come to it and you will look half-baked. If you are not presenting yourself as well and you do not know what you are doing, that is an issue.
Another thing you do not want to do is spend an inordinate amount of time on a blog. I see lawyers spending a lot of time on their blogs, but if they are not doing it right, they are probably not going to get any work from them.