We tell em for months not to take candy from strangers, and then we send them do an entire neighborhood full of strangers to ask politely from every stranger living there for...candy.
And once the realization hits, that this actually works....that strangers will give candy just for asking, the motivation just rises with that grin as they go to one.house.after.another. Til they just can't walk anymore, or until the bucket is too full. Between the church's trunk-or-treat, and Halloween the night after, the kid was in candy nirvana. And he gets it now. Not a baby anymore, no sir. He walks right up to those doors with a strong TRUNK-OR-TREAT (still confused by the magic words from the church carnival where we march from trunk to decorated trunk).
I took a picture of the gym, because I was there two nights in a row til all hours of the night helping create a dropped ceiling and making sure carnival games were set up. Too much work, but it looked cool, went well, and is over.