My UI

Here's a picture of my UI when everything's up and running:

The placement of a lot of my toon's abilities on their action bars doesn't overlap which each other. This means that when I press a button then in most cases it's a button for a specific alt to cast a specific spell.
But to simplify the controls to the point where I can use them, I overlap where-ever possible even if the spells are slightly unrelated. My main tanking/DPS abilities are also overlapped so I can spam them easily ^_^;;
Also, most of my abilities are macroed. As I can't give much time or thought to each different toon's rotations, a simple rotation set up with a /castsequence macro helps me get the best DPS that I can in the situation.

My tank's window occupies the whole of my first monitor, whilst each alt has to put up with just a quarter of my second display. I macro to the point where I can control most things my alts do with the keyboard, so the only thing I actually use their windows for is buffing, dialog boxes and inventory management.

You might notice that I'm ignoring my own advice and am tanking a paladin on /follow. He's actually got a strange spell rotation where he casts his two ranged attacks, then off-heals the tank until they come back off of cooldown.

It's a bit difficult to see which addons I'm running here, but the main ones are: Clique (for buffing and healing a specific toon in an emergancy), Automaton (quicker grouping/repairing) and Dealy Boss Mods.
You might have also spotted that I'm using Omen; it's there more out of habit than anything else, and not really needed for multi-boxing.
You can also see Xpadder down there; I'm a big fan of Xpadder, but that's nothing to do with multi-boxing ^_^;;