toothpaste and wrinkled paper

Over the past couple of days I have been working on a specific lesson with my daughter.  Here's the situation:

My girl gets it stuck in her mind that she desperately needs something.  It could be a piece of candy, a cup of hot chocolate before school, a specific water bottle, a shirt that needs to be laundered.  Whatever it is, if I say "no" then the words that come out of her mouth get worse and worse.  From whining to complaining to insulting me with the intention of saying the most hurtful thing she can think of!

So the other day I had her take a piece of paper and crumple it up.  Then I asked her to try and make it perfectly flat and unwrinkled again.  Impossible.  I explained that once the words come out of her mouth and hurt someone (hence the crumpled paper), you can't take those words back and make life just as it was.  Those words leave a bit of hurt on the person you insult (hence the wrinkled paper).  

Then my sister-in-law shared this one with me: If you squeeze a bunch of toothpaste out of its tube, you cannot shove it back inside.  Same with words.  Once the hurtful words come out of her mouth, my girl cannot put them back in again.  Saying "sorry" does not put the toothpaste back in the tube. 

What about you? What stories/methods/ techniques have you used to help your kiddo think before they speak?  Because after all the whining I heard tonight, I'm going to have to try to teach this concept again...

KC