I'll recommend the addon, Jamba, by Jafula.
Whatever you're boxing, it should be quite helpful to you.
It is in components, so you can disable the parts you don't like.

You'll need either hardware or a software multiplexer.
Some of the free options include: Auto Hot Key, Hot Key Net, and Octopus.
Some of the pay options include: Game Commander Pro, IS Boxer, and Keyclone.
I'll recommend IS Boxer as the best (imo), or HKN as the best free option.

If your software supports FTL, implement that.
IS:Boxer has it automatically.
As in, put Lightning Bolt on "1" for every toon, and they'll auto assist the active toon without any macros.
You can still do a castsequence macro if you want more then one spell at a time.

If your software doesn't support FTL or you cannot figure out how to implement it...
Go with a Focus based approach, as that is one of the easier methods.
If you are five boxing, have five macros, on the same keybind for every toon.
As in Shift F1 (on every toon) runs the macro:
/Focus [Target=ToonA]

That way, you can set up your DPS as:
/Cast [Target=FocusTarget] Whatever
Or go with the /castsequence, the same as with FTL.

And the other basic is:
/Follow [Target=Focus]