Well I know I am a victim of being hasty and making that nice weapon or armor and then forgetting to use powder of fortification on it before imbuing. Well here is a way to add durability to it after imbuing it. Although you always risk the chance to destroy it, but hey...
For weapons Enhance with:
Dull Copper + 50% Durability on Armor, +100% on weapons.
Shadow: +100% on Armor, +50% on weapons + 20% Cold for weapons.
Valorite: Armor +50% Durability.
I know this dont bring it up to 255, but its better than a 50-60 durability. Hope this helps and you will love the next one...