My personal method is to step in front of whoever is attempting to cut, and in a loud, clear, projecting voice state "Excuse me, the line starts back *there*," while pointing.
Now, I'm a large guy in all dimensions. I've never been seriously challenged after making my stand, beyond a bit of verbal face-saving. If someone were to push, though, I'd probably try to engage the rest of the crowd in public ridicule and possibly get security or management involved. I'm not looking to start a fight, after all.
Anecdote time: My wife and I were standing in line to see the latest Pirates movie recently. It wasn't opening day, but the theater still had the guide ropes up and people were waiting for almost an hour in line to get into the theater. We were near the front.
Some punks thought they'd be clever and wandered over to the entrance after hitting the concession stand. They did not get in line, but were hanging around "looking at posters" and "visiting the bathroom". It was pretty blatant what they were trying to do.
Anyway, I'd come straight from work and was still wearing my corporate clothes. I excused myself out of line, went around the center (this theater is laid out with the concessions and bathrooms in the center and the theaters on the outside), came up from behind, and in my best professional voice said, "Excuse me, folks, you'll need to wait in line if you're going to see Pirates. It starts back there."
They all tried to look innocent and one of them said, "But, uh, we're just waiting for our friend, she's in the bathroom."
Maintaining the "I work here" pose, I said, "Well, I'm afraid you'll need to wait out in the lobby. People can't be hanging out in the halls, I'm sorry."
With grumbles, they went back to the end of the line. I waited there until the theater opened, joined my wife in line, and thoroughly
enjoyed the shocked look the little shits directed at me when they realized that I didn't work there.
I tell ya, with the right attitude, people will assume the damndest things about you.