What is the best way to paint factory plastic?

Well, paint as in color, decorate, draw on, whatever. My siblings and I are thinking about getting an iPod Shuffle for our mom, along with a purple case (her favorite color). I was thinking of painting something on the case for her, but I’ve never had any luck painting on factory plastic. I usually use acrylic, but that flakes off if you even look at it wrong, and since this is going to be carried around and thrown in purses, I’d like something a little more resilient. Any suggestions on paints or techniques? I’m not limiting myself to paints, inks, pens, anything that could be used would be awesome.