I think it's fair to say that most professional programmers know that they should use source control, but, unless you work for a company that mandates it, you treat it like flossing or going to the gym. However, unlike your personal health and well-being, using source control is very important and can improve your life noticeably.
I'm a recent convert, in part due to Domino's historic hostility to proper source control. However, since 8.5.3 and its inclusion of the surprisingly good Source Control Enablement, there's no more excuse to slack. If you haven't already, read/watch the following things (and shame on you):
But beyond the basics of "make sure to check in changes with useful notes", I want to figure out the best way to do Domino development in this brave new source-controlled world. Mainly, I think I'm going to try to adopt a model similar to a shipping application: longstanding "write once" branches for significant versions, lengthy but temporary branches for development of the next version, throwaway branches for pie-in-the-sky ideas (like whenever I want to try to re-do my forum app's data access layer), and deployment via Source Control Enablement's "New Database From..." functionality.
That last part is huge, especially now that it's so easy to point an XPage app to a different database for its data access. I'm going to try more of an app/data separation in the future - I'm not sure I'll stick with it, but I think it could be very useful in deploying a new just-about-live version of an app alongside the "real" live one.
Regardless of the specific form my development process takes, I'm aiming to integrate source control thoroughly, to the point where NOT using source control feels "dirty", the same way that modifying a production app directly does. Fortunately, it doesn't take much to reach that point - just one instance of a client asking "is there any way we can go back to how it was working at the start of the week?" and being able to do it and you'll never look back.