Maintaining a successful forum is difficult, but if you use the correct methods to satisfy your members and motivate them in regular contests, discussions etc.
the job will not be as difficult.
Here are some tips which I would recommend you put into action to motivate your members and maintaining the forum activity.

• Improve the registration rate and activity rate to eliminate any sense of inactivity.
  How? Invite friends and family to register and post once in a while.
  
• Give incentives to members who are active at your forum.
  Rewards may include: Free advertisement, a custom group, profile/account enhancement, etc.
  Depending on your forum genre, you can provide a service.
  
• Requirements, don't hide all your resources from your members.
  Hide the "premium" resources from your regular members.
  Many admins make the wrong decision by making 25 post requirements for gfx requests, then another 10 posts to use these resources etc etc.
  This just encourages spam.
  I found that if you set one standard requirement to view all resources/access to services, it will improve the forum activity/less spam in general.
  
• Don't act GOD! Many administrators close topics, abuse power and intimidate members, obviously this will cause disputes and loss of activity.
  Learn to acknowledge members input.
  
• Moderation - Not too strict, not too lose.
  Keep it moderate.
  But definitely remove spam no matter what.
  
• Offer interesting titles to users, forum awards and other offers to keep people posting in your forum.
  

I hope this article helps you in maintaining a successful forum.
More tips will come soon in "Tips on Maintaining a Successful Forum Part 2".