My Main char was my Focus and assists were based off assisting the Focus in pvp and pve alike; I didn't change Focus at all(which was a problem when my main died and I tabbed over but I was still a tri-boxer then so no big loss)

I now use a macro based off targeting/assist:
/tar (target=Main, nodead), (target=Alt1, nodead), (same for alts 2 3 and 4), (target=Main){without the nodead tag it defaults to him if the entire group is dead so I can still corpse run, but uses the highest living alt as lead so I can alt-tab through them if I lose my main}
/follow
/assist
/stopcasting

That is my "Big button" courtesy of Zyzzfxzxczxfcs(or whatever his name was lol) and has worked quite well so far as a single button solution for both Assisting and Single same-key Lead-switching ;p