Love this topic...
I work from home, so being able to switch gears is critical for my sanity. Some of my colleagues are firing off emails at 2 a.m. and working 7 days a week. They're also the ones we get the email about... "so and so is pursuing opportunities outside of *company*. Please join us in wishing them well."
I multi-task all day long...bouncing between work, online play (like now), household and kid responsibilities...until it's time for work to end. I do that by the clock...and it's 5 p.m. unless I have a critical conference call. Then I will work until the end of that call, period. I will quickly jot down any tasks I'm responsible for or notes I
need for the next morning, then I'm done.
I never, ever, ever spend more time at work than a normal day...or I'll end up being one of those burned out colleagues. For me, longevity is the most important thing...and work is a marathon, not a sprint.
So....when it's time to quit for the day...I flag any emails that haven't been resolved from the day, I add new tasks to my "to do" list, and then I shut down my computer and close the lid. I do not go back to that desk until the next morning.
The "dividing" activity for me is always the same...dinner preparation. Whether I'm cooking or running to KFC, the act of getting dinner is the dividing line. Work stays on the other side, in it's own little mental compartment. I don't think about it, or generally talk about it, at all once dinner is underway. Once dinner is being prepared, it's all about home and kids and partner...whether that's helping with homework, watching TV, reading a book, whatever.
I'm the same on vacations and weekends. If you talked to me while I'm on a weekend or a vacation...you wouldn't even think I had a job.