I also use separate keys for healing. All my macros are something like that (at work so I cannot copy/paste, might be a bit wrong) :

/cast [target=focustargettarget,harm,nodead][target=focus,nodead][target=focustarget,help,nodead][target=player] Healing spell

Focus is of course my tank. The idea is that my healer will heal, in order of priority :
- The character that my tank's target is hitting (usually the tank himself if he is going his job, but it helps on mobs that like to change target)
- The tank, without further ado if he does not target a mob
- An ally/team character if my tank targets him
- The healer himself if everything else fails (usually that means the tank is dead)

I have 4 healing keys, all with the same targeting rules.
The first three ones are specific to the healer :
- Riptide
- Healing wave
- Earth shield (really needs to be refreshed ... when needed)
The fourth is common to all shamans :
- Lesser healing wave (that's usually my "oh s**t tank is about to die" button)

Then I have a fifth healing key with chain heals all applied to target=player. This is nice to recover from or heal through AOE damage, especially since it also heals the tank if he is in range.

A little trick that I find useful is to put a copy of the main DPS macro (in my case simple Lightning Bolt) on every healing key on the three DPS characters. That way, I don't don't lose too much dps even when I have to focus too much on healing.