This is where bike riding and running come in for me. Those are my standard "after work" activities anyway, so when quitting time comes around, I put on my shorts, grab my bike (or running shoes) and head to the trails. That, to me, demarcates that it is now "after work".

But, as noted in my other reply, weather doesn't always cooperate. But on a day like today, I may pick "going grocery shopping" as my line of demarcation. Worst case, I could get in my truck and just go drive around a little bit to achieve the same purpose.