It really depends on how many different classes you have.

If you have added a second class to tank with then you have two options.

1. use a castsequence macro on the tank that does the abilities you need to maintain aggro.

2. Unbind the 1-= keys and just click the abilities as and when you need them. Then you can concentrate on tanking, and just hit the number of the spell you cast on your alts. It takes a little while to get used to but isn't too difficult.

If you have multiple classes then you need to work out the core abilities you will need and map them to similar abilities.

For example is there a main attack you use all the time. E..g lightnight bolt on a shaman Wrath on a druid.

put them on the same key - take cast times in to consideration.

Alternatively you might try and castsequence attacks.

e.g

1 key gets a shaman to put lightning bolt.

on the warlock this might trigger a castsequence doing curses and dots.

If you have a dedicated healer I would seperate his heals off on to another set of keys, targetting different players with different levels.

(I've never had a dedicated healer on my team, so don't know how that would work.)

Unfortunately no one can really give you a setup, you have to work out what works for you.