For the forum:
Keep it active. Always answer any posts quickly and clearly. Try very hard to resist the urge to say, "That's not on topic" or "Please ask this in the proper forum" - if it needs to be moved, just move it. Prepare an area for off-topic or "chatting" posts, and let people know that it's there and they're more than welcome to use it. Post a few "tutorial" or "news" posts, to show people that they can post more than strict questions about the hosting service. Make sure that your rules are clear and concise. Make sure that, as traffic begins to increase, you have good moderators in place. If you personally know any of your clients, ask them if they might help you out by posting things in the support forums, even if they're outside your own strict support guidelines: design questions, web programming questions, what have you. Even if the answer might be to refer them to a forum or site elsewhere, if you have that URL, provide it to them and tell them why you think that site might help them.
Beyond that...it isn't easy getting clients to post on the support forums. Some people simply aren't comfortable using them.
Good luck!
