All good tips here, can't really add anything. Just practice, practice, practice. A good full day at the track and you can get it down pretty quick. These were taken towards the middle of my first panning practice a little over a year ago.
When I first got to the track I used 1/500th sec. That was a bit fast but allowed me to make sure I was getting sharp shots. Once I got my proper panning speed down for a given subject I lowered my shutter speed to allow the wheels to show some spin.
Iso 200 F8 1/160th sec @ 250mm