White vinegar, a bit of dishwashing liquid, and lots of hot water. It's most likely some kind of mineral deposit. If that fails, try some of the solutions you can find by searching Google:
http://www.google.com/search?hl=en&q...oogle+Sea rch
What's that commercial stuff, it comes in the yellow bottle, initials of some sort, like CRX or something, you can buy it at any grocery store...that's my place of last resort, and almost always works well!