There are a couple of refinements that you can make to slow-synch flash. Some flashes allow you to set them so that they fire at the end of the exposure not the beginning. This means that the flash exposure is, in effect, superimposed over the blur, not the other way around. You can also use colored gels to match the color temperature of the flash with that of the ambient light. This will make the picture look more balanced.
Adapted from Travel Photography by Steve Davey (Footprint, 2008; $30)_