you need to be specific. visualize the person or persons involved and the desired outcome, don't think about yourself at all. focus only on them. this being said, sometimes the caster can get tangled up in the spell. example, you heal someone of a sickness, but it was something they were suppose to suffer through as a lesson. you took this lesson away, so you will get sick because the universe/gods want to show you what you did wrong.
mind you, this doesn't happen that often, so long as you meditate before doing it so you're sure it's the right thing to do. as well, like i said, while casting, you are clear, precises, and focused on the person or persons. if you're casting a spell for someone else, you shouldn't have anything to do with the magic or energy apart from casting it in a direction.