I was wondering your techniques for having motion blur in the background with people and a person in the foreground with no blur. Also how would I work this with using an external flash. I want the person in the foreground to be well exposed without any blur. I also have control over the people movement in the background, like telling them when to move and how fast.

I know use a slow shutter speed, but I want to see if there is any other methods to have a perfect shot, what to avoid, and the perfect environment.