Your current members aren't going to stay forever.
You can optimize this a little. You can convert more newcomers into regulars. But every member will leave eventually.
The only communities that survive over the long-term are those that attract a steady source of new members. Don't leave growth to chance. Don't engage in one-off promotional strategies. A flood of members for a few days drowns out the existing culture.
Instead, set up multiple streams for new members to arrive (or inbound links that bring in traffic).
- Post several slides sharing the community's best advice on a topic on slideshare.
- Write to relevant, large, organizations and ask them to add a link to the community on their intranet page.
- Create and publish an eBook of the community's best advice/memorable stories and post this available to download and on the Kindle.
- Contact those that run relevant, large, events and ask them to add a link to the community too.
- Invite bloggers to mention the community in posts (or interview a blogger and ask them to link to this).
- Highlight questions on Quora and other sites members might want to respond to.
There are plenty more, you can come up with your own.
The goal, as you can get, is to create multiple streams through which members can arrive at the community. These are sustainable sources of traffic. The number of clicks doesn't drastically vary from one source to the next.
If you need new members (and you probably do), don't go for the quick promotional explosion. Go for a constant increase in new, inbound, links to the community. Create enough streams and you'll have a sustainable river of new members.