I just returned from the grocery store where some idiot entered the
express lane with an overflowing cart. I start off with "do you
realize that you're on the express lane?" She did. So why are you
here? That gets things rolling. She had kids with her, so I asked if
she thought that lessons in dishonesty and inconsideration were
valuable? She called me Mr Perfect, for which I thanked her. And so
on. In a supermarket that has a dozen lanes open, her sort is the
lowest form of life. The clerks are not allowed to say anything, so
the customers must, or else these idiots will just go on and on doing
this. Of course you'll be called all sorts of things, the least of
which is "douche-bag", but what do I care?

Another tactic, rather than direct confrontation, is to loudly count
their items as they place them on the belt. If they ask, you say that
you're just helping them because it is obvious that they are "slow".

If they have been "invited" by the clerk to bring all their junk to
the express lane, I just get the manager to open a lane for me.