Quick house rule that I want to use in my campaign - if someone (or even some of the monsters) is not actively defending himself (i.e. do not try to parry, dodge etc.), his AC is lowered by two steps (i.e. from AC 5 to AC 7). If he is not using any armor and not trying to avoid harm, he will be hit automatically.
I'm not sure if it will work for ambushes, backstabbing and similar actions. We will see.