TL;DR: Never found a good way to automate it; but nice tools.
While checking for ways to automate the creation of sequence diagrams I came across sequencediagram.org and really liked the ease of use and syntax.
Although the syntax is quite similar to the one of PlantUML, I really appreciate the idea to do just one thing in your browser. No client, no frills.
Another interesting one is mingrammer/diagrams.
I still lack a proper usecase, but the overall idea is nice and it is good to have a tool to chart cloud-based interaction.