Here are some of my ideas:
Easy and balanced way: Make them add +1 to the armor value.
Nice twist: Remove CCB 1 (or more if you like) from the opponents attacks.
Balance this by adding a special "Shield fighting" skill, before you characters gain this skill fighting with a shield will reduce any CCB and maybe even a CCB -1 stat.
Types of shields
Buckler: No CCB penalty, no training needed, no +1 to armor. Removes 1 from any enemy CCB attack.
Round shield: Need shield fighting skill or suffer a negative modifier, +1 to armor, 6+ Cover save. Removes 1 from any enemy CCB attack.
Tower shield (large shield): Need shield fighting skill, +1 to armor, 5+ cover save, Dexterity or Constitution requirement before it can be equipped as the weight cannot be handled just by anybody.