2) Triage!
Triage is a skill that is controversial among healers but it is vital. Triage is simply healing priority. While you'd think it's simple, it can be difficult to grasp. One of the most important lessons to learn is this: Healing is about keeping the group alive until success, not every single member. There will be situations where it is disadvantageous to keep certain people alive, because they will die again, or you are unable to leave your main target long enough to do so. This will cause quite a bit of anger. No one likes sitting around dead while you win a fight. Explain that you decided to make the choice you thought would ensure the win. Some people understand triage and will understand your viewpoint. Learning triage is an ongoing process. As you're playing, learn the situations where your intervention will be unhelpful. Some of them are easy and some of them are much less so. Know your limits. Know the limits of your party to the best of your ability.