Assign members with projects to retain them

If your forum is young and you have some regular members who seem to be loyal then try assigning them the task of helping out with special projects. They don't have to be given staff roles but maybe a nice colorful rank to show that they're standing apart from others could give them a little boost.

Some of the things we had early on at Christianity Haven were advisors and there were different things these people would do such as one guy would do Hospitality and make sure to message every newcomer or leave a visitor's message. We had a Foundations advisor who helped to set up the basics of the site and give the forums names. Another helped with technical things since he knew the software very well.

This builds a loyalty among your members and even though not all of our original advisors are helping out in that same capacity, they're all still active members at the site 4 years later.