This has always plagued me. Just when I think I'm doing alright, I blurt.
I've come to realize that those who know me are able to keep up. Those who don't know me may get a strange look on their face, so then I can tell I have to backtrack. I usually tell them I'm sorry for interrupting, had they finished? Or, I just ask them directly about whatever they were saying. Using excuse me helps, but I still can see later that I'm interrupting the flow. It's something I've gotten used to, I guess.
I heard a good one the other day, which is to keep checking, as in repeat back what they said, then once they acknowledge that you've heard them, try to switch.
One of my favorites is saying, "Well, this may be a bit off-topic, but..."
Usually, if the NT is dead set on maintaining their topic, they'll swing back to it after I interrupt. At this point, I may just say I have something I have to attend to and leave. hahahaha!