"By means of this device the Magic-User houses his life force in some inanimate object (even a rock) and attempts to possess the body of any other creature within 12" of his Magic Jar. The container for his life force must be within 3" of his body at the time the spell is pronounced. Possession of another body takes place when the creature in question fails to make its saving throw against magic. If the possessed body is destroyed, the spirit of the Magic-User returns to the Magic Jar, and from thence it may attempt another possession or return to the Magic-Users body. The spirit of the Magic-User can return to the Magic Jar at any time he so desires. Note that if the body of the Magic-User is destroyed the life force must remain in a possessed body or the Magic Jar. If the Magic-Jar is destroyed the Magic-User is totally annihilated."
-Men & Magic, p. 28,29
The main problem associated with this spell is that persons possessed by the magic-user will not, unless they too are a magic-user, have the proper cerebral cavities and cephalic distortions necessary to memorize spells. The possessing magic-user would have to start out with no spell ability in a new brain and bend his mind back into proper shape by leveling up.
Obviously the immortal magic-user prefers not to deal with this hassle if at all possible. Therefore, some method of permanent additional brain capacity in which to imprint spells is highly desirable. The following are two such items.
Affixable Ancillary Brains: Stacks of brains harvested from 11th level magic-users and alien psionic creatures are riveted together in a framework of bone. The magic user interfaces his neural strings to those of the additional brains, granting him one additional space for storing spells, enough additional hardware to employ several sets of new sensory organs should he choose to affix these to his person (sensory organs not included), and the capacity to learn one additional language.
IOUN stones: IOUN stones function as additional brain cavities, and are considered by many magic-users a far more elegant and aesthetic solution than appending additional brains to their own. Such a device allows a magic user to imprint spells within, whereupon they will hang suspended around the magic user’s head until the spell is cast. An unimprinted IOUN stone will absorb the first spell cast at its possessor, and then finishes its total gravitational implosion, permanently destroying the spell. If you use spell levels, each IOUN stone has a different color corresponding to the spell level they work for. If you don't use spell levels, each IOUN stone is a translucent white until imbued with spells whereupon they change color corresponding to spell stored within.