When there is a dispute over who is "next" in the masses that mob the counter, I leave it to the customers because anyone I would choose would be wrong. "Who was next?" This usually works well. Sometimes I KNOW someone is next and I have to step in and say "this person has been waiting." The amazing thing is that grown ups will just take cuts anyway. Like this:
There is a line. I establish that a certain person is next to have my attention/be rung up/whatever they are waiting for in LINE.
Someone comes up with a "question," usually prefaced with "quick."
As they take up my time with an increasingly complicated situation, I maintain eye contact with the person that I have said I would help next. Hopefully they understand my glances (this person is CRAZY! Can you believe what is happening??)
Then I either help the crazy as quickly as possible to get them out of everyone's way or pass them off to a co-worker when they become available.
I love it when the person that was waiting says something catty. ME-YOW!