I have it easy - I work in a nightclub. First time they do it, I tell them:
"It's much easier for me if you order all at once, I can make em all with
ease."
Second time they do it, if it's going on a tab, I make the initial order, put
the drinks on the tab, and move on to the next person without acknowledging
that they wanted to order more - it's Pavlovian, you have to train them to
place the whole order at once. If they're paying cash, I'll ask them straight
out "Is this the whole order?" If they say yes, I tell 'em the total and make
the rest. If not, I tell them I'll come back when they have the whole order
ready for me, and that they need to pay me for the initial order. Good
customer service sometimes relies on the fact that you have a good customer.