Always fill all of the slots on your
weapons and armor with materia even if you aren't
going to use them. Just like your characters,
the materia gain level through experience,
and the only way the get experience
is if you equip them.
You really must learn how to use materia
properly and experiment with various
combinations of weapons and materia.
Some weapons and armor will give experience
faster to your materia, but may be weaker
offensively. Find the balance that works best
for you.
