Our site is little, so little that I wont embarass myself by putting our stats, LOL.
Anyway, we started by offering goodie bags and surprise gifts related to our forum topic to new users. This worked out well all the way around because new members actually received something tangible that they could use, we got rid of inventory that we couldn't sell or things that vendors gave us for free (if you tell your vendors what your doing many will give you freebies to give away), and most of the members stuck around. The postage and cost of the items (if it came from stock) is a tax write off too. And, we did have a small issue with some hit and run posters, coming in to register for the free stuff and disappear, though we remedied that by ammending the policy to new, active users get a little something - and the goodies end at user #100. Then as a further reward we created a user group for the members that really stuck it out and have been super-dooper active (bascially making the board) and offered to them once per quarter an order where they could buy something at cost plus 10% plus shipping.
These both proved to be beneficial for us, as Google Ads broke us quickly and only brought us spammers. Pay Per click didn't do much justice either - so we were stuck. This past month we also started a promo of the best layout of the month (basically the best picture in the gallery) and the winner gets a biography page that will promote them, including links to their galleries and other stuff, which has really generated alot of competition, equaling much more activity within the board.
Maybe these little tid bits may be of help to others. In the meantime, I am going to keep watching to see what all others are doing for more tips that we can try too.