The first thing is to be sure that you're talking to the right person. That would be the one, who can make the decision to use your services. You should have some sort of claim to make that will capture that person's interest quickly. Try to solve a problem for them, so you'll need to find out what that problem might be by asking some questions, like "Have you used an online job posting service before? How did that work out?"
One issue with job services is whether they do any screening of applicants. If you can save clients looking to hire someone some time by not presenting them with unqualified/inexperienced applicants, that should interest them.