I've been wondering this myself somewhat, but I've not read all the magic rules yet so I don't know if it gets a mention or not. Looks like the answer is "not"...
For starters, personally I find the "my character has debilitating condition X so he can always draw on it for power!" idea is silly. If it's one of the character's aspects, expect to be compelled all the damn time to the point of uselessness. But anyway...
Consequences are one obvious route. You get hurt, scared, embarrassed or whatever and you can tag the consequence by saying "I draw on my fear for power" or any of the other ways Harry does it in the books.
Aspects placed by manoeuvres - sorry, manouvers - are the other clear method. Someone intimidates you, provokes you, smacks you literally or metaphorically across the face, hits you with the Lara Mega-Lust Bomb #1, whatever it is to put an aspect on you to their advantage and bam, you draw on the way that makes you feel to boost your magic. Another fine Harry trick.
Last option is to, I guess, place an aspect on yourself somehow. Since I can't imagine how or what you'd roll to make yourself angry or scared, and just saying "I'm angry now" and calling that an aspect is too easy, you could, say... make like a non-rolled declaration and spend a Fate Point to place "ANGRY!" on yourself. You get a (sort of) free tag (provided no one spots that you're angry and uses the free one first...) and you can boost your magic with it.