You can train it almost daily by rejecting people trying to (up-) sell you something and be it just in a restaurant declining to have desert or coffee after the meal.

Over here that latest gag in a bakery chain is that after ordering the loaf of bread you came for they don't ask "would you like anything else", but rather " what else will you have" (a bit difficult to translate) I give this person just a blank stare and say nothing. And I don't move or say anything before they give in. I know that they have to do it and I wouldn't get aggressive with them, but some small thing like this is a good training.