I have always traveled with other people, and we situate ourselves in the line so that people cannot get around us. Not in an obvious sort of way, but we kinda zig-zag ourselves along each side of the queue and don't let people shove past. Usually works like a charm. People try to nonchalantly squeeze past but when we don't let them, they typically just stop trying. We don't even have to say anything or look at them; we just continue our conversation and don't move.
Body language also makes a HUGE difference I have found. If you're just kinda lounging there, people are more like to push past; but if you stand up straight and tall people tend not to mess with you. If I see people coming up to try and cut I tend to either cross my arms across my chest or put them on my hips; makes all the difference especially if you're female!! (haha, sounds so intimidating!! I'm really not scary

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Obviously I wouldn't get into a fight or anything over line cutting; but usually these things are enough and the cutters don't even say anything to us- they just give up.