I echo the comments of the previous experts. As one SEO expert told me, search engine optimization is on-going exercise, not a one time event. This is the situation here, and I would separate you efforts between new and returning visitors. Sites in general that get a high number of visitors on a monthly basis usually have a core group of returning visitors that leverage it for content sharing or expertise on a regular basis.

In the case of returning visitors, you could have a forum/testimonials for people have who have taken the test, leveraged your service and done well. You could also look to start with other languages for content for other countries to provide a point of differentiation to your US only competitors. Blogging, linking to articles relevant to your user base, also curating news through sites like Paper.li or Rebelmouse provide an easy content option without much effort.

For new visitors, media exposure and references from top opinion leaders in the space can help. I would look to other portals in your industry or that attract the age group you are targeting and request the site owners to write a piece, send out a communique and/or reference your service to the targeted audience. Getting local media in your areas to report on your service can also provide exposure. I always focus on finding the free avenues first and when you find the highest referring sites, focus any ad dollars there to get the best bang for the buck.