In this image I used the “zoom effect” differently. Instead of changing the focal length steadily I changed it in four steps. To give you an example: If you are using a 28-135mm lens and a total exposure time of 30 seconds, you could start out at 28mm for about 7 seconds, move to 60mm for another 7 seconds, move to 100mm for 8 seconds and to 135mm for another 8 seconds.
Enjoy and keep on photographing!