I host a software forum and we periodically give free serials to the software at times plus some other software not too expensive either. Then user are emailed to know about this contest and participate and invite their friends and family to join. This has done a nice job without getting spammers too. Other forums I visit do the same, but as mentioned depends on your site and what it has to offer. You want to be unique but at the same time kind of have some redundant sections like PC help for example. I don't get much on members but get around 70+ a day not counting visitors. My site is only 3-4 years old.