Promotion, promotion, promotion! Your job site is what you put into it. The publishers who have the most success regularly talk about their job site. Has someone put a new job on your site? Include it in a blog post or mention it in an email to your users. The more you can do to make a job poster feel like they are receiving value from your site, the more likely they are to post.

Also, try giving out discount codes and creating job packages. Check out some more tips at the Job-a-matic blog.