I heard a good rule of thumb for any magic system involves:
-Something the magic CAN do
-Something the magic CAN'T do
-The COST of using it
(Sadly, I cannot recall who originally coined this.)
So for a basic example, say we're making a story about necromancers.
-The magic CAN allow you to raise a dead body and control it
-The magic CAN'T actually bring the person fully back from the dead
-The COST is some of your blood, more the longer you want to control it
Keeping these rules simple and common sense is usually good practice, and can quickly make a believable and balanced magic system as you build on them.