Christina,
I'm no expert on the subject and will be learning on the go. But, here is one I'd taken at an indoor rodeo where lighting was a challenge (and the first time ever I was shooting at an indoor rodeo) so it didn't take much to "design" the effect:
(Sony A55, Sony 16-50/2.8, at 50mm, f/2.8, 1/160s, ISO 1600)
I had AF-C on, and was tracking the head until it got to its max speed at about the middle of the arena. So, I was about 3x (or 2x, if we consider crop factor) the focal length in this case. However, it also appears to have froze the background a bit more than you may like. The challenge may also be in trying not to blur the legs out completely while getting a good streak in the background. I would say that 1/160s may be a good point to start, and go up to 1/300s, evaluating the results. The horses are likely to be going faster than my capture so the latter might work well.