Hi Katherine!
I'm in a similar position, working with a community that's now a year old. And the stickiness of the community as well as overall health are also two of my main challenges. Apart from things that have already been mentioned, one thing that has worked for me well (but are also a work-in-progress) is to celebrate either community-related or company related holidays. It helps a lot to bring variety to the otherwise business-oriented discussions. I.e. with Klaus, we sort of have it easy: cat-related holiday(s) is a perfect excuse to do something fun, sometimes we also connect it to a giveaway. I also keep occasions like Customer Service Week in mind and have already some polls, discussion rounds, etc, planned.
A big factor will be the functionality and reporting you have available to you on the platform you host your community on. Always always always be sure that the gamification tactics you select will compliment your overall community goals! Hope this helps.