Instructions
Visit the high priest at Alltrades Abbey. Change one of your characters to the mage vocation and change another one of your characters to the priest vocation. These are the two basic vocations that have the best magical skills. You should also change a character to the sage vocation if you have unlocked it by finishing quest 115, accessed by reading a book on the second floor of Gittingham Palace.
Fight battles in order to level up your priest, mage or sage. Every few levels, the character will learn a new spell. These new spells will get progressively stronger as your characters level up.
Place the majority of your ability points, which you get when you level up, into the spellcraft skill for the mage, the faith skill for the priest and the enlightenment skill for the sage. Both magical might (magical attack power) and magical mending (magical healing power) can be leveled up significantly this way.
Feed your mage character any seeds of sorcery and your priest character any seeds of therapeusis you come across in your travels through the world of "Dragon Quest IX." These items increase your magical might and magical mending skills respectively. Since the sage uses both healing and offensive magic, he can be fed both of these seeds.
Equip any armor or weapons that you find that raise magical power. These will give you a temporary boost until you find significantly better armor.