Contact your students via an interactive messaging system, such as Skype, Yahoo! Messenger or AIM. You can set office hours for the students to interact with you, or you can make discussion sessions mandatory. Students are more likely to engage in a chat session, where they get immediate feedback, than in a static discussion-board format.
Post video content in your online classes. Limit the videos to no more than 10 minutes to keep students' attention. Make content videos to help students with difficult sections of the class or post an introduction to the syllabus or other sections that students tend to skip.
Post regular announcements to guide students through the expected work for the week. Regular announcements set up expectations and keep students focused on the weekly tasks. Make sure that when you post an announcement, it's also sent to the students' email accounts.
Answer students' emails within 24 hours, if possible. If a student has to wait longer than this, his focus on the topic will wane.