Set a 'focus' and make your macros in such a way that they always assist the focus.
This way you can use the same macros on all characters and you can easily switch to them by setting a new focus. Hitting the focus button also triggers my Keyclone to swap windows.

This is my complete setup with an explanation on how this works:
http://www.dual-boxing.com/wiki/inde...aman_macro_set

In short:
- all current helper toons have focus to the main toon
- the main toon does not have focus to himself, so he KNOWS that he is the main and you can use this to improve your scripts (optional)
- all attack scripts are trying to attack the focustarget, otherwise they just attack as if the character is playing solo

[edit]

So in practice this is:

Press the focus button macro when you change the main character:
/clearfocus
/focus [modifier:ctrl,modifier:shift,target=Avarell] [modifier:ctrl, target=Willyam] [modifier:shift, target=Jeck] [target=Joeh]
Each client sets the focus, depending on the modifier (shift/control/both/none) that is pressed. They now know who to help.

(optional) Now clear the focus only on the main(active) window, dont replicate this key:
/clearfocus
Now start casting like this:
/assist [target=focus, exists]
/cast (spell)
which will first try to assist the focus (if this is set) and will cast afterwards.

or this:
/cast [target=focustarget, exists,harm] [ exists,harm] (spell)
Which will cast to the focustarget or if it the focustarget doesnt exist, then it will cast to any currently harmful existing target.

[edit] To conclude, with this system:
- the leader acts as a single player
- it's helping toons will help when there is a leader selected (through focus)
- removing the focus on any(or all) characters will make them behave as normal (non-assisting) toons

[edit - yeah another one]

Another way to accomplish the same is to not make the macros focustarget-based, but part1target-based. This would make the toons assist the partyleader. This is generall a bad idea if you want to use your group in a battleground or 10-man instance(or more) where you might not have leadership.