In Small Favor and other books, Harry "draws in energy from the environment" that he stores up large quantities and then uses later to power several of his spells, which mostly seem to be evocations (Forzare, for example). I haven't seen anything in the spellcasting rules that supports such a mechanic. Harry does this pretty casually without the usual caveats he spouts whenever he does something tricky with his magic, making it seem like this is no big deal.
In general, the rules seem to limit the number of evocation spells you can cast to two to four per scene (your Mental stress, which depends on your Conviction), plus whatever similar effects you might cast from enchanted items such as Harry's force rings. When those are exhausted you can start dipping into consequences, which gives a flavor similar to the books, where Harry talks about being tired after casting some spells.
The spell mechanics on p. 255 of Your Story speak of gathering power, but it is essentially limited to your Conviction, and is only used for the next spell. You can't suck in a bunch of power at no risk, and then meter it out spell by spell.
Obviously the ability to do this would greatly expand the spellcasting abilities of wizards who have time to prepare this way, and thus increase the potential for abuse of magic in the game. Novels by necessity work by different kinds of rules than RPGs. But the attraction of the Dresden Files RPG is that does a pretty decent job of making the RPG feel more like the novels than your average D&D clone.
Have I missed some rules that model this kind of power gathering? Does the new Paranet Papers book address this type of casting? Has anyone else come up with a set of homebrew rules that do this while limiting the potential for abuse?
Fate points can, in a sense, be spent to power thaumaturgy spells. Perhaps that might be the basis for a mechanic that allows wizards to pre-gather power for multiple spells or somehow manage to cast more spells after they're completely exhausted (which Harry does time and again).