What makes a law effective?

It has to be fair for everyone.

No, fairness does not guarantee effectiveness. The law must be easy to interpret (as enforcers and judges MUST interpret it), specific in its definitions, and harsh in its punishments. THEN it will have the effects its authors desired.

What makes a law effective is when the punishments are harsh life life sentences and things like that which can scare murders, so that they will not do it again (this is my personal answer).