Wow. I've spent nearly half an hour staring at this illusion (thanks, Drow Denizen!) and I still can't figure it out. Until Drow posted the three pictures side by side, the ballerina would only go clockwise for me. And even after looking at the pictures, I couldn't make the middle picture change direction. Only after I stared at the right picture for awhile and then back to the middle, did it suddenly--BAM!--change direction for me.
Now I can almost instantly make the middle picture alter direction, but only if I stare at the junction of her raised leg and abdomen. If I look at her hand, or her face, it's impossible.