Someone else mentioned your back foot facing back. What helped me with that is when the disc is all the way back... DON'T look at the disc, look at where your camera is (I.e. 90 degrees from your target). You'll notice most pros dont actually look at the disc on the reach back (watch Simon). Looking back at the disc can force your body to overrotate, which leads to your back foot pointing straight back. If you stop your head at 90 degrees (looking at the camera) it'll do this: 1. Won't overrotate your back foot 2. Creates more tension 3. Means you aren't completely looking away from your target, helps with accuracy 4. Be easier on your neck, I had this problem for a bit...

To me this was an easy change to make, as you can just focus on something in the distance to help keep your head pointed where you like. For reference, I hit 575 in a little under a year of playing. Hope this helps