There are thousands of tactics that will make the level of activity in the community go up.
Initiate a poll on a topical issue and e-mail all your members about it. Host an exciting event and invite members to join (their friends too!). Turn the most controversial discussions into sticky-threads.
But there is a difference between tactics that make the level of activity go up, and the strategies which make the level of activity stay up.
The problem with the above is two-fold. First, these tactics always take time. You will always have to spend your time hosting/initiating these. This isn't a single intervention, it's something new added to your work load.
Second, members get tired of the same things. You can't repeat the same trick forever. Eventually the activity declines again. You can't always host more interesting polls, exciting events, or feature even more controversial discussions.
It's always better, over the long term, to develop a sense of community. To forever push your community towards greater familiarity with each other. To use the transferable community elements within your community strategy.
This doesn't make the level of activity go up, it keeps the level of activity going up.