Pointing iTunes to a different folder.

[Gareth]

Or the really ghetto version is to rename your local iTunes folder (I use "iTunes local"), then have an alias "iTunes" pointing to your iTunes library on the external drive. Then rename them when you want to switch (I rename the alias "iTunes remote" and then rename the "iTunes local" folder back to "iTunes").

That's what I do, I don't do it that often which is why it works for me.