The latest post of Ostarys' Tactics is very useful for running spellcasters in the DFRPG:
Spellcasters: Managing Energy Points
It's great advice and very useful for people running spellcasters who aren't really conversant with the tricks and methods of the trade.
It does leave out the issue of "spells on," however.
"A spell that costs 2 looks twice as expensive than a spell that costs 1, but if your skill is 15-19, you can cast the second one all day long, but the first one only about a dozen times. Abuse free spells, they are your best trait."
Very, very true - and important.
But one little addendum:
Remember "Spells On."
Every spell that counts as a "Spell On" may be free to cast, but the only person you're spamming with them is yourself. Flaming Weapon on 10 guys is 0 to maintain at skill 15, but it's -9 to cast on the last guy and -10 to all of your other spells.
Offensive spells are generally superior for casting over and over. Frostbite. Low-point Missile spells. Mental Stun and Stun. Spells you can cast, see what happens, and move on.
Don't get so caught up in "free" that you forget about cumulative penalties. Your overall ability to function is limited by multiple spells on, but not by free spells you deal out over and over to disrupt foes.