Store what your enemies don't expect you to. If he's ready for a quaketrap, store fballs and suprise him. Or what about 2 quakes and a fball, soften them up then surprise with a change of tactics in the middle of it all.
Work effectively, well, sure, I die all the time :)
When you log your mage, what spells do you store?
For me the question is almost always: Should I store fbs or quakes?
Some guys go with mixtures, why? Does it work effectivly?