Since I've been hammering on this, I have an example I'd like to throw into the mix.
We're in mid-exchange, and the black-hearted warlock is throwing a blast of psychic brain buggery at the intrepid warden trying to bring him to justice. The warlock rolls his spell, nails his control and winds up hitting our warden with a seriously decent effort. For the warden's defensive effort he comes up short by 1, and there's no fate points left for him to use to bring to bear any of his aspects. He can't maneuver, basically he's bent over the mental table.
Can he declare that, due to his extensive training and experience fighting these foes, that his mind is hardened against such intrusion, and create a temporary aspect of, say, IRON MIND? And then tag it for his defense? That would give him that +2 he needs to defeat the attack.
Now lets assume that you can't declare aspects on yourself, what's to stop one of the warden's allies from making that declaration? After all, they take no time, aren't actions by the characters, etc. I'm disinclined to allow it, myself, but I see a potential here and I'm curious, indulge me, please.