When shopping with your child or a child in your charge, and you see something they would like or be interested in (a toy, a game, a fun clothing accessory, etc.), please do not go out of your way to show it to the child and encourage them with "Look, this would go great with your (something the child already has)," or so forth. Then when they are all hyped up to get the item, turn them down, claiming perhaps they can get it for an upcoming birthday or if a gift-giving holiday is close by. Tell them it's going to be part of their birthday or Christmas, just go ahead and get it right then and there, because if they're sad they don't get it, it's only yourself to blame.
I don't have children, thus I can't speak from that experience, but I've seen the above happen with a friend and her youngest daughter and it made me squirm. If I'd had the money then I would have thought about getting what the little girl wanted, and maybe I'd be in the wrong since it wasn't my child, but it was just an awkward situation to me.