What helped me be able to do an illusion is think of it as a one handed cartwheel with a quick spin in the middle. That's what my gymn teammate told me, and I got it, and she's awesome at Illlusions..she even has one in her floor routine!! She's starting to work them on beam as well.
a great drill I ahve used to teach illusions are to have the gymnast hold onto a hanging rope (like one use you for rope climbing exercises in the gym) as you snap down into the needle and spin. It really helps you maintain balance without using your hands.