I'm fairly new to doing anything more then dual boxing which you only have to switch characters when the main dies so there is probably a better way. I have all of the non-mains set to assist or target "party1" which is the group leader, if you see your main is going down promote one of the non-mains.

In my case I have my B character setup to jump right into this role, if B and the main both go down it gets rough real quick, at that point I'm scrambling to promote someone else and grab the backup mouse and more then likely just running as fighting is about impossible. Keep in mind this is the first time leveling 4 characters and I haven't spent time on pvp macros or much more then just blasting mobs one or two at a time. I'm assuming at 70 I will spend a ton of time getting my pvp and kiting type macros tweaked.

simple example

/assist party1
/cast Lighting Bolt