If you're using Keyclone and are willing to setup hotstrings, that's the best assist method.
There are several threads on this, along with the initial (long and detailed) thread.

I'd personally recommend either a focus based or leadership based system.
Mainly because they increase your power dramatically, and are relatively easy to set up.
Any toon can lead as well as any other toon, so if your -current- main goes down, switching to any other toon alive and you're still playing.

The focus system is easiest, but you lose the ability to use /focus in the game.
Basically change all your targeted spells... from /assist previous leader... -to- /assist focus..
and keep the cast or castsequences the same.
Then have one focus switch key for each toon on your team. IE, if you have four toons, then each toon has four focus keys.
So you have a focus key per toon, when you switch to that toon every team member makes that toon their focus.
Whatever the active toon targets, each member (active toon included) has them as the focus, and assists.
Very easy and effective.






If you want to retain the /focus ability, example pvp burst a non-targeted toon or pve crowd control etc...
Use the Leadership assist, which requires paging.
In wow, if you press Shift 1, you're on bar 1. Shift 2 puts you on bar 2. These two bars need to be clear of everything else.
The bars are virtually identical, except every toon has an assist version of their bar on bar 1 and no form of assist on bar 2.
When you're toon is a follower/slave, they're on bar 1. When they lead, they're on bar 2.
The assist format is either /cast [target=party1target] ..... -or- (Line 1) /assist [target=party1]... (Line 2) /cast Spell...
The two liner is good for pve, because each slave targets the leaders target. The one-line is good for pvp, as the spell hits the correct target, but the slaves never have to target it.
Your switch macro's are in two forms. Every toon has one button which is their own macro and this is different from the other toons buttons.
For the "other toons", the macro is a two line:

/script PromoteToLeader("Toon Name")
/changeactionbar 1

Every toon except the newly promoted leader clicks this key.
IE, if your leader change keys are Shift-F1 (me), Shift-F2 (Shm 2), Shift-F3 (Shm 3), Shift-F4 (Shm 4), then you'd have a variation of this macro on keybinds Shift-F2 through Shift-F4, with the corresponding toon's name on each macro.
Your leader's key (Shift-F1) for this character has:

/changeactionbar 2

If you're using Jafula's addon Jamba (highly recommended), add a second line here:
..... /jamba setmeasmaster all .....
Every toon has one copy of this macro, on the keybind which makes them the leader. So toon 2 has this on Shift-F2, toon 3 on Shift-F3, or whatever your keybinds are.
So when you change to toon 1 is the leader, it puts toon 1 on hotbar two and all other toons attempt to promote the active toon to party leader and go to bar 1, which is the assist bar.

The reasoning is... pressing F2 in warcraft is the same thing as /target party1. For the non-leaders you get the party leader.
So the non-leader's can assist party1 (party leader) and have everything work fine.
The party leader presses F2 and they get the second party member not themselves. So they cannot assist party1.





Either of these assist methods will let any toon lead the group as well as any other toon.
So you can lead with one toon and switch to any other if the first goes down.
You can even lead with one, come to a stop, switch to a secondary with the previous leader (now slave) slightly in front and likely to be targeted first.
This would save you the minimal time wasted switching to a new lead.