Your base level of magicka increases with your intellect. So choose
intellect for some of your level ups.
Anything that will fortify magicka (or, if it works like Morrowind,
intellect) will temporarily boost your magicka. Mixing a potion of
Fortify magicka, (spell, etc) will work. In Morrowind, fortifying
magicka or intellect did not necessarily let you cast spells you
previously could not (high power spells). Don't know if this is also
true of Oblivion. Note that if you have something to restore your
magicka, then casting a fortify magicka can be beneficial (even if it
cost more magicka to cast the fortify than you get boosted).