She's at an age where she'll be desperate to be seen as a 'big girl', so make comments about how big girls don't behave like that and that she's making herself look like a baby. I guess it's all about finding her achilles heel! :0)

Tell her santa wont come if shes been a naughty girl and taking her knickers off! or any other person like the easter bunny...

Also, maybe letting her pick out her own in pretty patterns or her favorite character will make her more proud of them and she'll want to wear them more. Of course, then she may want to show everyone . . . but I suppose one battle at a time? :)