If you want some well thought out Parrying rules for 3.5, take a look at
Skull & Bones by Green Ronin and Adamant Entertainment.
In a nutshell,
Give up an attack from you next turn* to make an opposed attack roll to parry an attack. Even if you fail the opposed roll, you opponent must hit your AC. You cannot parry more times than the number of attacks you would normally have with a full-attack action.
*This means that you MUST take some sort of attack action on your next turn, and you must decide what bonuses and penalties (from Feats, etc.) apply to the attack roll when you make the parry. Characters with high BAB or weilding two-weapons may make several parries, but they MUST take a full-attack the next round.
Example 1: The enemy attacks and you decide to parry a single attack. The next round, you take a normal Attack Action, but do not attack (the attack was used to previously parry). You have only a single Move-Equivalent Action for this round.
Example 2: The enemy attacks and you decide to parry a single attack. The next round, you take a full-attack action, but make one less attack than normal (the attack was used to previously parry). Aside from attacking, you can only take a 5-foot Step for this round.
Example 3: The enemy attacks and you decide to parry a several attacks. The next round, you take a full-attack action, but make one less attack than normal for every time you parried (the attacks wre used to previously parry). Aside from attacking (if you had any attacks left), you can only take a 5-foot Step for this round.
Skull & Bones also has several feats to enhance the effects of parrying.
Ascii King said:
I would lalow it as well, but the group I play with take turns DM'ing. For this reason they don't allow anything that isn't in the rules.
That makes it tough.
Another house rule that I've used, if your group is willing to accept it, is to allow character to use Attacks of Opportunity to make opposed attack rolls for parrying... Taking Combat Reflexes allows you to parry more than once per round. It's simple and it's easy, and let's people do something with those rarely-used AoOs.