Teaching children patience ...?

I am having issues teaching my three and a half year old daughter patience. If she asks for something and I can't get it for her right away she gets angry, starts crying and freaks out. I have tried to explain to her that she needs so wait until I am able to get her chocolate milk or I can't read her a book because I am driving etc. and she repeats over and over until it gets be to either get it for her or ends up in a time out for losing her temper and not being patient. Does anyone have any ideas? I need help!