What is the best way to display money on a website that will be available in multiple languages? I can think on many ways, but can't see the best one.

The easiest branch is using always the same currency. But this also creates a problem on how to present this information to the user in a clear way that we are talking about dollars. So, should I use $ 100.00, USD 100.00 or 100 USD ?

Other alternative is converting to the local currency on the fly. While the benefits of this approach are obvious, wont be weird that every time the user enters on the site the price for the products change?