I'm using something like :
/assist [target=focus,help][target=party1]

This way, when i use a 5-man group (99% of the time), i just have one touch to press to change the leader.
3 key in one on a G15 macro (yeah, i know it's bad, but...) :
the first is a macro /jamba setmeasmaster all with a do-not-pass-key on keyclone.
the second is a macro who set my page bar to the second action bar on all character
and the third set my page bar to one, it's a do-not-pass-key so it's only for my main.

This way, i have only one key to press, and my current windows is my new main character.

And on battlegroups or raid, i just use the focus.