We force a Google AdSense banner on the sites, and oversell the single server which we stuff these accounts on like heck
Of course, that server is a bit up and down, down at the moment.
We also allow people to get "instant" activation for $1, and remove the ads (but remain on that server) for $15 per year.
It is absolutely impossible not to oversell free hosting. Spaceports, I recall, often had to add new HDDs due to space issues.