One thing we did a few years ago when we found that a customer didn't use revision control was to bring in a server. A small PC with Linux, Subversion and Trac. We not only could explain the benefits of RCS, but the customer could see changes, att issues, get them resolved etc. When the job was done, the customer kept the machine.

I occasionally bump into old customers and many still run the same server. All of them are today using revision control systems.

So basically we didn't just provide a tech solution, but also brought in methodology and free tools to implement that methodology.