Around corners
Firing missiles from around corners is a standard part of both fantasy and actual combat. But fair's fair--if someone can shoot from around a corner, they can be shot at.
The houserule I use is that you can shoot from around the corner, gaining 50% cover. You can't move before and after to get 100% cover barring special things like haste or shot on the run. I chose 50% because that's the cover bonus you get from fighting from around a tree, or around a corner in melee. To use this tactic, you need to have the movement to put you in the 5' square just past the corner (i.e. the one out into the open if you're coming from behind the corner, or the one behind the corner if you're coming from out in the open). You actually place your figure halfway between the two squares to signify the position, but for all other purposes (future movement, attacks against you, sharing spaces, etc.), you're considered to be in the square behind the corner.
My players and I have been satisfied with this for archer shoot-outs, and while it's never come up for spellcasting, I don't see why the same thing wouldn't work.