1- Heal mid-close range with no scope when you can
2- It's okay to hit a squishy target once or twice (Pharah for example). No need to secure a kill, you'd rather be healing a team mate in that time. Plus landing 2 shots on them makes them play less aggressively because they want to be healed
3- Use nade offensively only when you want to punish an over extending enemy and at least 2 allies are focusing them
4-Practice quick scoping
5-Tanks are the easiest to heal so utilize on that