My follow macro has a /cleartarget, i use /focus 'mymainchar' on alts and /clearfocus on my main and my damage spells are all as follows:

/cast [target=focustarget, nodead, harm, exists] Spell ; Spell

So all my alts always have their target cleared, while I can swap main without worrying about it. Just need to press my follow button whenever I swap and have the old main target something to clear their target if for whatever reason I have too.

Might help you? *shrug*