Once you've set-up the forum as your online syllabus, make sure to give students a clear idea of how often you'll use that syllabus and how much they should depend on it. Let them know if it will be kept up-to-date, if it will only be used for important and long-term messages, and what information they'll be able to access. You might only publish assignments for the next class period, or perhaps a tentative 2-week schedule. Emphasize that this won't totally replace handouts, so they should still keep track of the print copies; you can never be entirely sure if access to the forum will be available. As the semester progresses, announce to students that you've posted something they can then access on the forum.