The trick when mixing classes is nothing more than finding similar spells or actions and then putting them on the same key on each character. Conversely, if you only want one character to do an action, for example having your shaman heal, you can put the heal on a key that is empty on the other characters.

I had 2 shammies and 2 mages, and one shaman was the designated heal bot, so I had 4 keys bound to SHIFT1- SHIFT4 and each one was simply a

/target toonname

/cast healspell

You can also add modifiers to cast different levels of similar spells, especially for healing.

Don't get too eager to write macros where they aren't necessary, headaches ensue.