I've seen this approach cure introverts many times... Get a job in a quiet pub/bar. One day a week will do. I'm deadly serious. The first day you'll be hellishly nervous, but you'll quickly get used to the very limited conversation you have to have with each customer. Soon you'll realise it's really easy and you were able to do it all along. A pub is better than a shop as there is the opportunity to extend conversations with customers. Plus you soon realise that they say the same thing over and over again, so the conversations are easy (if a little dull).