There's no easy answer, but I don't believe justice is served with overly specific laws and mandatory sentences.

However, leaving the scene is a separate offense to whatever was involved in the accident, so one step would be to mandate that the offenses are sentenced independently and that must be consecutive. This would preserve judicial latitude, but provide for more severe punishment.

As I said earlier, there should also be a presumption of guilt for DUI if there's evidence to support it. In other words, leaving the scene and not driving directly to a police station would strip someone who can be shown to have been drinking (in any amount) of the defense that a blood test might have provided. But note, that there would still need to be some form of evidence to support the inference.