When I started building WordPress sites I actually thought my job was done once I handed off the site to the client; the code is written, it is theirs now. I suppose that worked: I was able to pay my bills, but didn’t see many of those clients again.
Then I started providing some documentation and training for clients. Yes, it was a bit of a pain, but curiously enough they kept coming back for more work, and not just more training work, also development work.
I firmly believe that upping my game by providing training and documentation is the reason clients started coming back. Expanding my level of service created a great way for me to earn more, both with my hourly rate and also by having more items to charge for.