Shields are tragically undervalued in Dungeons & Dragons and its offspring, so it is my duty to correct this oversight by borrowing the wisdom of OSR brethren and tinkering further with it. The wisdom I am borrowing is from Shield Saves and Shields Shall Be Splintered. Without further ado:
All shields improve armor class by 1 vs. mêlée attacks.
All shields improve armor class by 1, 2, or 4 vs. missile attacks. (Small shields improve by 1, medium shields improve by 2, and large shields improve by 4.)
All shields enable the wielder to attempt to block successful attacks after damage has been rolled. The wielder chooses which successful attack to block and makes a saving throw. Fighters roll their most favorable save; non-fighters roll their least favorable save. If the saving throw is successful, the blow was blocked and the damage is deflected. Otherwise, the damage is sustained.
All shields allow the wielder to attempt to block one successful mêlée attack per round. In addition, the wielder may attempt to block missile attacks as follows: small shields may block one missile per round, medium shields may block two missiles per round, and large shields may block four missiles per round.
The defender may opt to sacrifice the shield instead of rolling. The defender suffers no damage, but the shield is destroyed by the attack. This tactic may be used against area effect attacks such as dragon breath, fireballs, lightning bolts, etc.
[Edit: See Concise Shield Rules for my current perspective.]