In addition to basic employment conditions, the agency worker has the right, during an assignment, to be informed by the hirer of any relevant vacant posts with the hirer.

BBC: Agency workers' rights explained

Although there is no magic answer, let me suggest one basic guideline: Whether you are commissioning or receiving the work, spend extra time shaping an assignment at the beginning.

FORBES: Why Smart People Can Be Stupid About Asking For Help

The basic process is this: Demand Media collects explicit search terms people use on their sites and then assignment desks content which heavily features those terms.

FORBES: Data Driven Editorial