Back in 2009 I wrote a post that didn't get a lot of notice at the time, but that we used in our Ancient Academy game and I think worked fairly well: In Defence of the Shield.
A character with a shield may roll a single d4 for one attack causing damage on them per round. If the roll is greater than the amount of damage from the attack, the shield blocks it and no hit points are lost. If the roll is equal or less than the damage, the attack causes the regular amount of hit point loss.
A character’s shield roll may be “given” to a nearby ally if both characters are fighting the same opponent (or group of opponents attacking from the same direction). Characters must declare that they are fighting next to another character before they may share their shield rolls in this way. A single character may not make use of more than 2 additional shield rolls per round – those from the characters fighting to their left and right.
Magical shields allow a player to roll a dice one step larger for each “plus” they provide. A Shield +1 rolls a d6, +2 a d8 and +3 a d10.
If you'd rather use something a bit simpler, here's a revised AC chart based on JD's initial suggestion:
Unarmored: AC 9
Leather: AC 8
Chainmail: AC 7
Shield only: AC 6
Leather + Shield: AC 5
Chain + Shield: AC 4 (same)
Plate: AC 3 (same)
Plate + Shield: AC 2 (same)
Only the lighter armor types change their AC value as historically the shield played a diminished role once heavy plate armor became common in the late middle-ages.
Both of these house rules will let you make shields a bit more important in your game, without moving things too far away from the original combat systems.