Slow Shutter Speed techniques, Foreground still, background blur

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.

I've done a lot of intentional motion blur shooting. If you use a flash and pan with your main subject, it's pretty easy to get a clean shot. You set your exposure for the ambient light with a slow shutter speed - maybe one second. Make sure the ambient exposure is a little on the dark side, maybe 1 stop underexposed. Then you set your flash output to light your subject. If you pan with your subject and the flash output is good, you'll have a sharp subject and everything in the background will be blurred and slightly darker.

Can you describe what you have in mind? If I can get a clearer picture of what you want, I can better help you achieve it. And what kind of equipment do you plan to use? That helps, too.