@ Mono:

Most multiboxers that set up their system properly, have their macros in such a way that any of their characters can be played with. It usually only requires 1 button press to inform all the characters which is the main character (e.g. setting the focus). This same button sometimes activates a screen/window swap also.
After that, macros can be set up like:
/cast [target=focustarget,exists,harm] [exists,harm] Something
This way you cast something to the focus target if it exists and is harmful. When this character is just acting 'normally'(without a focus/focustarget), then it will just cast the spell as usual.

@ Frosty:

It's better to use /focus unless you only do PvE. In PvP(battlegrounds), you can't guarantee that one of your characters is the leader.