The shield as present in most DND-esque RPGs is not the most useful of devices typically improving ones armor class by a single point. Thus reducing the chance of getting hit by 5%.
In real life, even semi-real life (LARP) the shield is far more effective then it is made to look in most games.
The typical weakness of shields over rates the effectiveness of non-shield fighting styles. But from personal experience the fellow with a shield has a decided advantage over someone fighting with two weapons, a two handed weapon or caught off guard with but a single handed weapon.
I myself have been prone to fairly complicated shield variants in the past to expand the effectiveness of shields to be more in line with real life effectiveness as I have experienced and studied but often it's just too much. Basing variants off existing rules and mechanics seems to be the way to go.
Advantage to maneuver:
The shield can be used as a battering ram improvign ones odds to get a lethal blow in against a foe. If one has a STR bonus they are allowed to apply that bonus to initiative when engaged in hand to hand combat to reflect their ability to bash aside body and weapon with the shield.
Advantage to defense:
The shield is much better at aiding ones defenses then it is typically credited in RPGs and an easy method to show that is to negate AC penalties when so armed with a shield if clumsy and added then again or the agile above what is already awarded.
So Kelfdin the mighty is armed with sword and a shield, has a DEX of 8 and a STR of 16 in LL he would find himself to be AC 5 normally. With the mods listed he would be AC 4 with an initiative bonus of 2 when engaged with a foe.
Gerald the Bold is armed with a mace and has a shield for defense, he has a Dex of 17 and a STR of 12 in LL he would normally be AC 6 (shield and dex) but under these rules he'd find himself to be AC 4 agaisnt foes he could block with his shield.
Further variants: The impact of shield size. For thsoe wishing to differentiatie shields further
Buckler... good 1 foe, no initiative bonus, not usable against missile weapons.
Small... good vs 2 foes,no initiative bonus, only usable 1 thrown weapon.
Medium... good vs 3 foes, initiative bonus, usable against 2 thrown or ranged weapons
Large... good vs 3 foes, initiative bonus, usable against missile weapons
Tower...good against any foes to front, initiative bonus, usable against missile weapons
possible prices for shields:
Buckler 4 gp, small shield 7 gp, medium shield 12 gp, large shield 20 gp, tower shield 30 gp.