My children are young - almost 6 and 3. My 6 year old has been adding to her list for months. LOL - everytime she says "Oh, I want that" I write it on a piece of paper in my purse and add it to her list for her. It works well for us because it's basically the same stuff she wants over and over - so I pretend to just keep adding to her list. :-)
When she gets older, I don't know what we'll do. I'm just enjoying these young years! LOL