I've had great success in using the Discussion board to promote learning. Here are some tips I've learned. Feel free to add your tips or disagree with any of mine.
#1) Require students to post
I've heard many students say "I thought the discussion board was stupid, but it turned out to be really helpful." Unless it's required they won't even look, but once they see it they'll start to use it and benefit from it. Some students think they're good enough that they don't need it... well great, you can go in there and help answer other students questions.
#2) No limits on the posts
It's ok to vent about how much you hate the class. It's ok to talk about questions you don't understand. It's ok to post a meme. It's even ok to just say "I'm posting here to get the points". You can start a new topic or reply to someone else's. It shows you knew about the resource, how to use it, and you were participating in a dialog with other students.
#3) It's ok to post answers
If there's no limits then that means you can actually post answers to homework questions. Let's be real: the answers get passed around anyway. With randomized questions in WebAssign sharing answers means explaining how to get the answer.... we call that teaching. The exception would be exam questions, but for anything else don't worry that you'll get in trouble for posting something.
#4) Have a place for funny comments
If any post is allowed then of course you're going to get a lot of posts that are off topic. That's ok, but start a forum or topic thread for non-class related posts. That way students that want to get real information don't have to swim through a discussion about Stranger Things.
#5) Create a topic thread for each homework question
If you don't then you'll have eight different topics all saying "Can someone explain why the z-score on question 3 is negative?" And as soon as you answer the eighth one someone will create a new topic thread "Can someone explain why question 3 has a negative z-score?" Train them to look for the topic on question 3 first.
#6) Thank them for posting
If someone replies to a student's question with a good answer make a point to tell them how awesome that is. Tell students "Many of you have been responding to others and helping them - you guys really rock". Encourage them to take pride in giving answers and helping others. Soon you'll create a culture of students teaching each other.
#7) Don't open boards until the homework is available
You want the students all working on (and talking about) the homework at the same time. That way the homework is fresh in people's minds, and there's more students looking at the discussion board each day. Don't let them post on a homework they haven't even started yet.
#8) Lock discussion forums for old homeworks
You don't want students to be asking questions when no one is reading it. It also helps them keep up with the class. They can still go see what was posted before, but locking stops them from trying to speak in a forum where no one will ever see it.
#9) Understand that class size matters
If you have lots of students then you'll have lots of help available when someone posts a question. If you have few students then you might require more frequent posting. The smaller the class the more important it will be that you check the board frequently. If there's only a few students you might feel a greater need to have them post about homework questions specifically
#10) Know the grading tricks
I wish WebAssign could automatically give students a grade for posting. Since it doesn't you (with your TA's) need to track who is posting. A couple of tips:
There's my top 10 discussion board tips. If you have others please chime in!