You can make it a membership site, you can run ads for colleges and training programs, other sponsored ads, corporate affiliations/partnerships, guest blogging… ads are the best since job boards tend to have a high number of page views per session. You can also do fremium upgrades like professional resume writing or specialized recruitment/ job finder by a real person. Also you charge people posting jobs a certain amount either per month or per post, and offer upgrades to get their posting listed at the top of the page for high visibility, also charge to view a certain amount of resumes and searches per month. So lots of ways.