Can't move the post or delete it.
For thoroughness, you can set the Main Assist as your focus and use a macro like:
/cast [@focustarget,harm,nodead][] SpellToCast
To explain that: you are casting SpellToCast on the target of your focus if that target is an enemy (harm) and not dead. If those conditions are not met, then the spell is cast on your current target (that's what the [] does - it always returns true, so it always passes). I use this on instead of Parkuss' assist macro, as he posted here, since I can maintain a target as well. I find it more useful as a ranged DPS to set the main baddie as my target and the MA as my focus. That way, I am always attacking something, even if the MA is slacking. ;)
Also, I have a macro tutorial in the Macro forum. Feel free to check them out. I'll post more in there as I get the chance.