Well, depending on the mage, I guess that the circumstances for learning new spells or getting new abilities would be different. For a holder type mage, you could either have your tools go through some sort of evolution, making them stronger, or you could simply find stronger magic artifacts.

For a caster mage, I think that it would be both a physical and mental thing. Although not the same, I would like to look at Chakra from Naruto to explain this. You'd have to know how to channel and mold the chakra/magic power inside of you in order to create new spells. You could also learn spells from tomes, as well as physical teaching from others. molding the energy differently can create variations of a possibly already developed spell, and continuing to deviate from the source material by making changes to every single variant can create a new spell altogether.

Also, the fire mage is in.