I've tried being organized, laying out things a week in advance in order to not be last minute about what I will take to a con. It never works.
Whatever I've laid out, a week, a day, or even half a day before will become my most hated items of apparel when the time comes to actually put them into a suitcase. "What was I thinking? Those pants make me look a mile wide!" "And that necklace never hangs right." "Those shoes are sure to hurt after an hour."
Better to wait until an hour before. Forced choices tend to become efficient ones: these pants never wrinkle, this necklace goes with anything, and shoes don't matter anyway if the pants are long enough. And if I keep smiling, no one will care if I end up with one taupe stocking and one suntan.