Request a desired item  

What to do in common scenarios

The person points at or tries to take the item.

Hold or place the items in such a way that they can't easily take them and so that even if they point it is not clear which one they are choosing (once you know - and the person knows you know - which one they want simply now asking them to repeat what you had hoped they would have requested is much less powerful for learning).

The person always asks for the last thing you said (e.g. always asking for a crisp):

Try reversing the order of the choice you give them;

If they don't seem to care which one you give them, give them a choice of a desirable item and a boring item - saying the boring item last - then respond to their request for the boring item. To stop frustration, you might need to quickly follow this up with another go where you only offer them one item (the desirable item);

Encourage them to point to a picture to show you which one they want - then reinforce this by signing and saying the word.

General strategies

Model the asking and requesting with someone (e.g. other staff) to show what you are hoping people to do.