You gain Mastery Experience from killing mobs, though you gain Mastery Experience significantly slower than standard exp. However, unlike standard experience, the amount required to gain a Mastery Point is static and as you grow in levels, you will find yourself gaining mastery points before you gain an additional level.
Certain Quests will also grant Mastery Points.
There are incredibly rare items, called Mastery Shards, that can drop from mobs. If you gather 4 Mastery Shards and bring them to a crafter, they can be forged into a Mastery Crystal which can be traded to special NPCs, found in major cities, who will grants you a Mastery Point in exchange. Mastery Crystals can be traded to other players. Note: Mobs must be a certain level before they will begin to have a chance to drop Mastery Shards.