Know your niche.
Knowing your niche is especially important if you're blogging with the intention of utilizing it for professional purposes. Remember that a blog is a publication. When selecting a topic/theme for your blog, select one broad enough to provide a wealth of content possibilities and narrow enough that it does not become unweildy.
Plot out a plan for posts of about a month. This plan should be mostly topical and thematic--an overview. Then stick to it. People like variety but they also like to visit sites which have source value. You want your readers to know that they can come to your blog and get the latest news, information, opinion, etc. on your topic.