When I was running a PVE group (now I'm running 5 shamans for PVP), I used a very good example that someone told me on these forums. I set up all my tanking and DPS buttons along with /cast [target=focustarget] Fireball and had my tank be the focus. Then, for healing, I used the keypad for the following...

0 - Follow on all toons
1- target main tank for healer
2 - target toon #2 for healer
3 - target toon #3 for healer
5- target toon #4 for healer
6 - target healer for healer

4 - Cast Greater Heal (I did this because its above the "target main tank" button, and that combo is used the most)
7- Cast Renew
8 - Cast Flash Heal
9 - cast Prayer of Healing
* - Cast Shield
+ - Spread macro for all toons.

The idea with this, is that you can quickly target anyone in the PVE group, and cast a heal on them. what happens, is you start crushing buttons with your left hand to DPS while healing with your right hand. Makes it a "little" tough to control mobs when they run away because you cant use the mouse as fast.... but I liked the setup.

Just my 2c.