I want to make an emanation spell not hit allies, specifically the 2nd-level Sor/Wiz spell Darsson's fiery furnace (Shining South 46). How can I do that?
The spellguard rings from Complete Mage come in pairs, and make the wearer of one immune to the spells of the wearer of the other. They’re not terribly expensive (4,000 gp), but you can only render as many allies immune as you have ring slots—and that means not wearing any other rings, and on top of that those allies also need to each give up a ring slot. Worse, they only function three times per day.
Note that it is possible to get up to five ring slots on a typical humanoid character: the hand of glory neck slot item grants one for 8,000 gp, and the Extra Rings feat from Eberron Campaign Setting grants two more (but requires Forge Ring and caster level 12th).
Probably not the only or best way to do it, but it’s something to get your started.