Well, you can hold contest/prize draws for members that have been here long enough/have enough posts to encourage other to join the site...
If memory serves correctly we did have that, but it was scrapped due to a shitty economy and lack of people signing up. Sometimes, free stuff isn't much of a real benefit to gain more participation, look at coupons. A lot of people could save a shit load of money, but people throw them out all the time.
I think jcmaz was referring to the "better contribution level, better chances at winning" idea
I think that a consistently rewarding benefit, like access to post a thread in the BST section dictated by participation and valued loyalty is the best way to gain more users who feel as passionate about this website as we do.