Implementing recurring billing & maximizing revenue from existing hosting clients

I host a small group of sites for clients I have worked with at some point in time during the last few years. I have not been doing a great job of tracking or collecting hosting fees. I am working to put this information back in order after a recent, pretty severe server issue had me taking a closer look at the domains I now host. This got me thinking about a few things which can be summed up in these questions:

How do you handle recurring billing?
(for example, a yearly hosting charge)

Once you already have the client, how do you maximize the revenue you can bring in from them?