Make a mental note about how much max hp each of your party members have and try to remember roughly your cure I and cure II would cure, that would help you not to over cure and conserve your mp on each heal spell you cast.

I use cure I 90% of the time, 10 % cure II, never use cure III. to be a good whm you need to be like what Synesthetic said, know the patterns of the boss, proactive to the damages.