After participating in the derail, I will often "copy reply" the original post. Just to remind us where we are.If you feel yourself about to hit reply but plan to type something that's going to derail (or further derail) the thread, simply copy and past the original post in here, tag the poster and put your off topic conversation here.
Most of the "derailed" conversation is very informational and entertaining. Sort of like being in the bar with yer buds.