Listen to mp3s, podcasts, and the news.
We live in such a digital age; even if you think you don't have native speakers at your disposal, you actually do. Scientific American, CBC, BBC and Australia's ABC Radio are great mp3s to get started with, but there's also a million podcasts out there and handfuls of news stations, too. And the best part is that these people generally speak clearly and have pretty generic accents.