James Farmer is compiling a list of "what not to do" when using blogs in education. My suggestion: Do not use blogs to replace writing or reader-response journals. If the only goal is to get students to write online what they would otherwise put in their notebooks, it's probably not worth the hassle. Blogs can do much, much more. Use blogs to enhance personal journals. Take advantage of the community-building potential. Let students work as a group of individual writers.
Do not mark individual entries. Develop a more holistic way of looking at student work.
... and one more thing that (I found) can have a very positive impact:
Use your own blog to comment on what the students are doing. Abstain from evaluative comments. Situate yourself as a participant, a reader - not a teacher - and the class bloggers will flourish.