To be honest; I don't know what you can do. I faced a similar situation with EaWH not so long ago - I looked at AoE3H (and in this specific case, GBH) and looked for what made them
them. Then I tried to make posts and such at EaWH that were different in nature to those at GBH/AoE3H so that all of a sudden, EaWH offers something different. I don't want to appear big-headed, but activity at EaWH is picking up as of late (allthough, this is in part down to the new expansion) - so it shows that such things work. What you need to do, IMHO, is try to be different. I have a few suggestions (it's worth bearing in mind that I've been at HG since 2000, and despite my relatively poor record - I feel I have a fair idea of what needs to be done - all IMHO of course);
1) Close the OT thread. This serves as a huge drain on forum creativity. Whilst it's a good way to siphen away spam at an active Heaven, here all it does is stop people posting their thoughts as new threads. More threads = more choice = more diverse replies = more activity. This is good, yes?
2) You'll need to replace the OT Thread with something similar for the time being, in order to "ween" the forum away from the AoE3H mould. This would be an excellent chance to hold a "Forum Survivor" game. It would play out much like a Mafia without sides, where the content of the thread would be forumers stating their case for winning. You could hold small challenges during the course of the game (ie photoshop, quiz etc) with an immunity award from being voted away. The thread itself can be posted in by anyone, player or specator - and would become, in essence an OT thread with a purpose. You could perhaps have the prize as, say a custom title of "RoLF Survivor Winner" or something, it would be a nice incentive for people to participate.
3) Work hard on the scenario/game sections to bring in more designers and RoLF newbies - they will bolster the other forumers currently here.
4) Try to create threads that offer healthy debate, but are not focused on any of the following; Current events, rights of women, religion, politics, war on terror, Irseal etc. Try to get good content debate threads that are lighthearted, or simplistic enough for the more inexperienced forumer.
5) Create more uncommon stock threads; for example an "RoLFH Advise Column" thread or an photoshop competition. Also, try to create threads that incorperate a wider range of forumers - try to hold something involving MSPaint or Notepad etc; programmes that everyone will have access too. Why not a "Best 400 character figure created in notepad" thread? Make good use of gimmicks, but don't overuse them as you could attract the wrong type of forumer or worse, alienate the ones you allready have.
6) Make an effort to spread the word that RoLFH is a good forum to join. I know perhaps this is unethical, but if you enter some chatrooms/other forums under another alias and advirtise this place. People are inquisitive and tend to follow links - draw them here, then keep them here.
7) Create more threads, but, instead of looking to other Heavens for inspiration - look to other forums entirely. See what makes the huge forums of the internet tick, and try to emulate certain threads here.
The most important point of all, and I cannot stress this enough, is steer this forum well away from the AoE3H mould as you simply cannot compete with it. If you offer a similar atmosphere and posting style, people will flock to the more active Heaven. Diversity is your ally. ...Hope that helped?