An integral part of becoming a Lich is the creation of the phylactery in which the character stores his soul. The only way to get rid of a Lich for sure is to destroy its phylactery. Unless its phylactery is located and destroyed, a Lich can rejuvenate after it is killed.
Each Lich must create its own phylactery by using the Craft Wondrous Item feat. The character must be able to cast spells and have a caster level of 11th or higher. The phylactery costs 120,000 gp to create and has a caster level equal to that of its creator at the time of creation.
The most common form of phylactery is a sealed metal box containing strips of parchment on which magical phrases have been transcribed. The box is Tiny and has 40 hit points, hardness 20, and a break DC of 40.
Other forms of phylacteries can exist, such as rings, amulets, or similar items.