There is seldom a perfect textbook available for a course. Expect to develop supplements to cover obvious textbook weaknesses before the course begins, as well as adding additional materials as the course proceeds and the students discover additional areas needing more information. Generally for an online course, these materials will be web-based, and should therefore be as concise as possible. These materials will contain errors. Not only that, students will expect those errors corrected, or at least explained, immediately if not sooner.
Personal contact is the most important key to success.
The more personal contact that online students have with their instructor and with each other, the more likely they are to succeed in class. With a bulletin board, instructors often find that students will answer one another's questions before the instructor even sees the message. While asynchronous communications are key, many students want synchronous, real-time communication as well. One of the most useful things an online instructor can provide is online office hours. Using chat rooms, IRC, and Instant Messaging capabilities, instructors and students can have virtual study sessions and dialog in real-time.