I set my tank as focus of everyone.

I reserve U I O P for healing.

U = Heal the main tank. /cast [target=focus] heal spell
I = heal self (if healer) except tank.
O = heal my target; or target's target. /cast [target=focustarget,help] heal spell; /cast [target=focustargettarget,help] heal spell;
P = AoE Healing.

Ofcourse my main healer has those filled.

For DPS
U = is the same as 1 or 2: /cast [target=focustarget, harm, nodead] nuke spell
I = any agro reducing, of self preserving skills. Ice Barrier, Water Shield.

If I need more heals for an encouter, I would have a 2nd healer cast heal on tank for the U key.