Model expected behavior. If the discussion starts getting off track, refocus. If a distracting topic appears that is generating interest, use what means you can to keep this separately labeled. Suggest that students look deeper into topics when applicable. Occasionally, summarize where the discussion has been.
Instructors can encourage students who get stuck on a problem to ask for help on the list.
To help students understand proper netiquette, provide them with examples of appropriate and inappropriate electronic communications. Students will then have more realistic expectations for responses. Some level of reluctance on the part of the students often emerges. Students need incentives or bonuses for participation in electronic discussion, particularly when their classes meet in person. Repeat instructions often.