Just like in terms of content creation or what?
A mixture of content creation with rewards for active/engaged members. Even Coca Cola with it's age, still make promotional games a
constant.
There are pretty well advanced addons of "Credits", "Badges", etc.. Any forum owner can use those tools to give more than Badges, etc.. Give real tangible things... etc..
I mean, forum owners does not have to convert forums into paid to post as a main point of attraction, but it surely lift people motivation to get real incentive from your community.
Good content is not enough to keep most people engaged or attract new members. There are free good content everywhere, you are competing with too many others for the same thing (good content) Add something else spicy

Example: $100 monthly contest to do X activity, etc... For some people $100 means a nice incentive, for others means eating for a month. So, don't devalue the impact of $100. Instead spending that on a shiny software or latest gadgets, spend it with your people...