What runes, masteries, items, or any other aspect of the game can help defend against targets that deal true damage?
If the true damage is your only concern, you should build health items and health regen, as well as Spell vamp or Lifesteal. Also, you could use crowd control effects like Stun or knock-ups to keep the enemy from being able to hit you. Short of all that, there's not much you can do against True Damage. Also, Exhaust does NOT reduce True Damage.
It depends on the kind of true damage that's being dealt and the champion dealing it.
For example, against olaf and irelia, you generally want more hp as well as AS reduction items like randuins or frozen heart, which incidentally reduces the physical portion of their attacks as well. These two champions are difficult to control in fights so either focus them down or kite them.
Against vayne, who deals % health true damage, get AS reduction items. Since vayne is relatively fragile, you can also aim to shut her out of fights by focusing her, either getting in her face and forcing her to leave, or use cc effects on her and kite if you don't have the chance to kill.
You can't reduce true damage, but getting more hp, hp regen, and lifesteal/spell vamp helps against it, since buying resistances will not help reduce damage, but those stats will make the damage have less of an impact.
Pure health like warmogs negate true damage. However if the true damage is like Vayne, based off total health, then getting attack speed debuff like Frozen Heart, Randuins, and Exhaust will negate a lot of the damage you take from them. The only item that builds against true damage in a way is Zhonya's Hourglass, but that is only for the 2 second active ability.