How do you respond by getting cut off or replaced in a line?

Just wondering, how do you respond or react to being “cut in line” by other people? Say if you can clearly see a large line of customers, and someone tries to “Barge” or “squirm” their way in line? What do you usually do or say to those people?

If I’m busy, I point them to the back and say “Line starts there, ends down there buddy.” What about you?