I have 100 Intelligence and Willpower and casting destruction spells at Expert level drain my magicka so fast it makes it nearly impossible to kill things because I lose magicka so quickly. Anyone know of a way to get around this?

Custom make spells, try command at level or 4 secs ( up to 25 ) as long as you don't wear armour it'll work at any level.
Another way chain spells, Weakness to magicka 100% 5 secs on touch, followed by weakness to shock 100% for 5 secs, then shock damage 25 points will kill most creatures, fast and quite cheaply too.

Self made magicka potions, Blue steel mushrooms and flax seeds, go along roads find them brew potions, almost a constant supply.

Mod the game, L.A.M.E is a great version for this, just at the moment its not on TeSnexus unfortunatly.

My mage does a couple of things, to keep that ole magicka full:

1. Restore Magicka Potions, as Madoc pointed out. Starting at about Expert level in Alchemy, I can make better restore magicka potions than the game can provide. These boost your magicka over time, rather than instantly. At Master Level, I can cast spells at the very tippy top of my magicka pool, and three seconds later, cast it again, as my restore magicka potion is still pumping magicka back into me.

2. Welkynd Stones. Use one of these for an instant full recharge of magicka.

3. My mage wears street clothes, and has kept two of his clothing slots for Fortify Magicka +50 Sigil stones, for the hood and wrist irons that I saved from the tutorial dungeon. The wrist irons weigh nothing, and do not count as armor so you can keep that 100% spell effectiveness. So that would be an extra pool of 100 Magicka over my Altmer, Mage Birthsign. The other clothing slots are reserved for elemental shields, or you could go for broke and add Fortify Magicka sigil stones on all five clothing slots, for an extra 250 Magicka over your pool, however, you would have no defenses at all.

4. Weakness to Magic and chaining spells, allows you to cast a relatively inexpensive (magicka-wise) spell, and some damage spells, to significantly hurt your enemy.