Always count your mana before casting, your team mates should never be flaming you for healing the enemy.
Always stay at the back during fights, never get initiated on.
Practice your combos; you can do E - Q channel - E - Q release if you're more than 500 units range.
Early false promise if your carry is in deep ♥♥♥♥.
On allies you can purge the disarm and keep the healing that way you can heal mid fight.