Unfortunately, we can't all speak at the same time. When I'm interrupted, if I'm focused enough to continue and the interruption doesn't bring up an obvious error, I apologize, take back the mic and continue my monologue.
If I'm not focused enough that I lose my train of thought as I was interrupted, I take notes and go back to it at a later point in the conversation. It is very important that you are persistent.
One thing that helps me focus is to have my notes in front of me before the conversation begins, so I can continue making my points or interrupt someone else with one of my points.