Do you provide free hosting for sites, right? Then you can place advertisement on the sites hosted on your server..
Your best bet would be advertisements and sell ad space. Maybe have some affiliate offer type of thing going?
There is not a whole lot of options get a decent site, include tutorials things to attract hits. Offer direct advertisment, google adsense is iffy but can work as well I do mix of both. Also provide a good service and you will get donations somewhat consistently however expect alot of work for not much pay off.
Perhaps you could try '6 Months free' Model. Its both free for your customer and not entirely free for you.
Put adsense ads and hosting affiliate ads on your main site and try to get as much traffic to your site as possible. You can also create a paid hosting site and try to upsell your paid packages.