Thanks also to anne for the link to this article: The Fourth International Symposium on ELT in China: Theoretical and Practical Applications of Emergent Technology in ELT Classrooms; How the Blog Can Change English Language Teaching
There's some good advice here, such as:
"The first step for teachers is to try out blogging for themselves. In this scenario, teachers cannot expect their students to do something that they are not willing to do themselves. Each teacher will have his or her own reasons for starting a blog. These reasons must be clear or the blog will not serve the learner’s language development. The blogs can be as general as a journal of daily thoughts, or be more specific in nature. The teacher should prepare beforehand, an activity that will be a catalyst for the language learner’s needs. "