When making rules and/or setting limits with kids it is vital that they make sense to the kids. "Because I said so!" is difficult to avoid but not a helpful way to speak to your kid. I absolultely believe in being firm but it is also important to be fair, consistent and predictable. Not easy I know but the alternative causes more problems and takes more time in the long run. I work with many kids who just don't know what to expect from mom and dad which creates an unpredicable environment which in turn creates anxitey.
When setting limits make the punishment fit the crime and make it time limited. Also of importance is to not dole out punishment in the heat of anger, take a little time to cool off and engage your logical brain before the punishment is rendered. Once the punishment is given make darn sure you follow through on it otherwise you will lose credibility. Just imagine how you would like for your boss to treat you and act accordingly with your kids.