Investigate the environmentEdit
First, determine the likely attack bonus of your opponent. This step is crucial; the difference between being 15 above the opponent's attack bonus and being 20 above is often the difference between life and death for an AC build. As an example, we can use the 60 attack bonus used above, which is a value well within reach on virtually any PvP server. Once the attack bonus is known, add 20 to it, or 25 if the value is only estimated. That is the AC your character must reach to be safe. Now we must go about reaching that armor class.
To maximize a character's AC, first look at the armor class page, and review the types of bonuses and how to acquire them. Most bonuses to AC do not stack, and thus are typically granted by one's items. However, dodge bonuses stack to a maximum of +20, and certain bonuses stack indefinitely. These two types of bonuses are the ones which are most important (on most servers) for an AC build. There is no need to cover each bonus and its applications here, as they each have their own pages, but as an example, consider a dexterity-based bard/pale master. This character would get bonuses from the pale master's bone skin and tumble at all times. Furthermore, the character could use a bard song or cast spells like mage armor, haste, and epic mage armor to increase their AC even more. There are a multitude of combinations to suit any character build; typically, a high-AC character will have one or two classes devoted to maintaining a high armor class, with the remaining class or two taking care of the main purpose of the character.
Finally, fine-tune your character's armor class. Maybe you ended up with a 90 AC (good job!) but you are only going to fight an opponent with a 50 attack bonus. Or perhaps you couldn't get your AC high enough because you want to take weapon master, and that took away too many levels and feats. Just keep trying different combinations until you end up with a result you like.
This is the more common route for builds that are not based around strength. The goal here is to simply accept that your character will be hit more or less every time, and to focus on avoiding damage or other negative effects. Immunity to critical hits is crucial for this strategy, either through pale master or an item. Damage reduction, damage resistance, and damage immunity can also be used. The standard — but hardly optimized — build of this type is a pale master/dwarven defender, using epic energy resistance and epic damage reduction combined with dwarven defender damage reduction and the critical hit immunity from the pale master to render the character virtually invincible.