I generally use the Common Law type things, but also I do this:
In General there is a 'rule of law', but that rule can be put aside or modified by the local ruler depending on his charisma and personal power (either wealth, force of arms, magical aptitude, etc). In other words, men rule instead of laws. Thankfully for most people, wizards and priests tend not to become rulers; they're generally too busy with their prayers or spell research to much bother with it.
You'll have Royal Law and Common Law; ie, there are different sets of laws for what social class you are. What is a death penalty for a commoner is a slap on the wrist to an aristocrat. Many professions police their own, which means Common Law might not apply to a wizard, a priest, or a member of the more powerful trade/craft guilds.
Towns have their own set of laws; they generally adopt much of what handful of laws the King makes. They can be more strict but seldom more lenient; all things being equal, though, as long as the local rulers get their taxes in on time, don't try to become King themselves, or don't go publicly overboard to the point of inciting rebellions, the King will leave them alone to govern as they wish.