Enough attention has been given to weaponry lately that certainly armours deserve some like consideration. To wit, we shall casually observe some rules–old and new–for shields, helmets and mails; equipments donned by hopeful heroes seeking some protection against the monstrosities of the Underworld.
There is no need to reinvent the wheel here. With shields, we must note that the rules for Man-to-Man Combat in Chainmail allow one so furnished a superior deterrent against missile attack, particularly when combined with suits of armour. Strapping on a shield with plate armour or chain mail increases your protection against ranged weaponry by bounds (compared to the jejune static +1 of the "Alternative Combat System" found in Volume I). Alternately, we must also note that shields in Chainmail are easily obviated by particular weaponry. Battle axes have a nasty habit of cutting through such impediments, while flails can usually find their way around edge of the shield. Similarly, shields do little to absorb the weight of maces and other blunt strikes.
Regarding magical shields, it is important to recall that the armour class bonus for these items does not stack with the bonus of magical armour. Even when the shield's bonus is superior, it is only taken into account 1 in 3 times (see page 31 of Volume II). Perhaps this can be considered the same odds for the shield to catch a non-lethal projectile as well, instead of using armour class (a rotten tomato thrown by an angry peasant, for example).
If you want more cinematic shields, I recommend Trollsmyth's "Shields Shall be Splintered!" in its most basic form (i.e. mundane shields can be sacrificed to negate a hit, after the damage has been rolled). Magical shields can, perhaps, absorb a number of hits for "free" each session equal to their bonus (after which, they splinter!).
These are included in Original Dungeons & Dragons without any explicit rules for their use. Volume II implies that magical helms are hit 10% of the time. For what it's worth, I prefer the Advanced Dungeons & Dragons rule whereby helmets are considered in the overall armour class and attackers will direct attacks against unprotected heads 1 in 6 times. If you are using Chainmail, this is easily represented by levying any attack roll that comes up doubles against the "no armour" column of the Man-to-Man Melee Table (this is also 1 in 6 times, with no particular weighting on strike probability).
Otherwise, helmets would give contextual protection against falling rocks, green slime dropping from the ceiling and so on. They should also limit vision and awareness, making such fighters easier to ambush.
Like those for helmets, the rules for armour are actually fairly scant in Original Dungeons & Dragons. Volume I implies that a more heavily armoured combatant moves more slowly (a holdover from Chainmail), although instead of using categories of armour ("Light Foot," "Heavy Foot" and "Armoured Foot"), the referee is apparently meant to use weight of equipment. To this effect, a warrior with chain mail, sword and shield could easily fall into the fastest category (12") while carrying extra gear into combat could quickly slow her down to the slower categories (9" and 6" movement a turn). Elves and Hobbits have the same 12" base movement as humans, according to Chainmail, while Dwarves halve this.
If you don't use encumbrance rules, I recommend simply allowing chain-type armour to subtract 3" from movement and plate mail to subtract 6" from movement. Either boost dwarf movement up to 9" to absorb this, or just disallow demihumans from wearing plate armour (incidentally, this is what I do in my campaigns).
In his micro-RPG inspired by OD&D, Searchers of the Unknown, Nicolas Dessaux makes armour class inversely proportional to both movement and initiative bonus. This is a clever way to truly represent how sluggish a warrior is in full armour. A simple way to represent this in Chainmail is to allow the combatant with the faster movement the title of "attacker" when two foes charge at each other. This gives the lighter-armoured opponent a little initiative in striking order, without displacing the importance of weapon class and the initiative roll.
Many others have lamented the oddities that have always plagued movement rates in Dungeons & Dragons. For my part, I like the simplicity of converting outdoor yards to indoor feet, but Gary's figures are still quite baffling. While Volume III allows only two moves per turn (that is, 240 yards in 10 minutes), Chainmail does a little better and allows one move per round (120 yards in 1 minute). Arguably, the latter rate is still too slow, and was likely intended to account for things like keeping formation, waiting on battlefield commands and other delays that do not factor into the small scale skirmishes of man-to-man combat. In my games, I allow two moves per round (in place of, and immediately following, each missile fire). Thus, an unencumbered fighter can run (240 yards per round, or 8 miles per hour) or walk (120 yards per round, or 4 miles per hour). An encumbered Dwarf would move about a yard per second, which also seems right. This is all keeping in mind that close combat range is 3".