One easy way to limit the size of the backlog is to let the community itself vote on which bugs and features you should be working on, then add a little editorial selection and take the top (n) to work on. That's what we do at Stack Exchange, anyway:

http://meta.stackoverflow.com/?tab=bugs http://meta.stackoverflow.com/?tab=requests

I find that the community appreciates being involved in these decisions, and often (but not always†) has good ideas of what you should be prioritizing and what issues are true pain points.

† http://www.codinghorror.com/blog/2012/02/listen-to-your-comm...