I cannot tell you how many times my daughter has come home from school missing buttons from her clothing, and of course, she has no idea where that button may be! :p
So, from now on, I always place a thin layer of clear nail polish on her buttons to keep them in place--the polish hardens and 'glues' the threads to the buttons!
I have even used this tip on a few of my blouses that have a some loose threads! ;)