[Nyororin]

If you already paid for multiple re-entry, then there is nothing more you need to do.
If you just have a long term visa without re-entry, however, you will have to go to the local immigration bureau to pay them for a little stamp that says your visa will not expire when you leave the country.

By the way, coming from America you can indeed get a multiple entry visa. It`s an option on the Japanese side, and as far as I know has nothing to do with where you`re from.