Search the community for articles on "Gatekeeper" and "Traffic Cop" campaigns. The webinar below can probably get you started.
https://community.marketo.com/MarketoArticle?id=kA050000000KytvCAC
And get some case studies from this presentation from the 2012 user summit:
https://community.marketo.com/MarketoArticle?id=kA050000000L61gCAC
At the user summit, Marketo annouced an easier way to accomplish exactly what you're trying to do, but the availablily was stated as a vague "rolling out over the next few months"