It's all timing on your part plus I use Cybersync wireless triggers. You leave the camera in bulb mode, so you can open the shutter for however long you want. Then they do their thing. At the end of the set, they remain still, I trigger the flash by hand with a remote trigger, then i close the shutter.
I used a cable release. YOu should get one for your camera too..so you don't have to hold the shutter button down during all that time. The exposures on these ranged from 10 to 20 seconds.
So this was done in a completely dark room so you don't get weird light. For focusing, I used a Canon ST-E2 Infrared trigger. Although it's not triggering anything, it emitts that AF assist light so I can autofocus in the dark. YOu can also use any canon flash that emits that red beam.
If you shoot Nikon, you can use CLS, or pocketwizards.