I probably shouldn't admit it, but I tease my daughter . . . hehehe. Remember the show 'Mister Roger's Neighborhood'? There was a character called Meow Meow Kitty and she would use the word "meow" every third word or so. So I imitate her in a whiney voice when my daughter complains - so she can hear how it sounds. For example, she will complain that someone ate all the ice cream and she didn't get her fair share. I whine back, 'Meow meow not fair meow! Meow meow didn't get any ice cream meow!' It's quite a lot of fun really . . . hehehe.
She can be sooooo negative and I tell her, 'Jeez! You are so negative! Think of the bright side! I want you to give me 3 positive things about this situation - see if you can.' I keep working with her and she gets a little better as time goes on.