Believe it or not I actually do very little flash work at night. Most of the time I use it during the day to remove harsh shadows and bring up the color in my subjects. Those same principles still apply at night but it might seem a little counter intuitive considering that a flash has one major purpose, pointing out light. Kind of need that to shoot at night.
The biggest challenge at night while hand holding, not doing star trails or anything else with a tripod but actually working in the field, is shutter speed. How do you get a faster shutter? Open up the lens, dial in minus exposure compensation in the camera body, or increase ISO. All three of those things will change the shutter speed. Since you’re most likely dialed in positive in the flash you need to really think about the rest of those numbers in order to shoot quickly while getting a sharp image. This Yellow Perch, a native Montana species, was photographed with the D5, 24-70 AF-S, SB-5000 with -1.0 exp comp in the body, +1.0 in the flash and ISO 1600. Just enough to get that pop of light needed to highlight another of Montana’s species.