Twilight is a wonderful time for making pictures. With a solid tripod and a long exposure, you can compose outstanding images in the soft light. For even better results, consider adding a flash to the mix. A flash illuminates the silhouetted shapes and the dark areas that you'd get when shooting toward the sunset's afterglow. With multiple flash bursts, you can light up a wider area or create pools of light to emphasize particular parts of your composition.
Working with flash at twilight isn't difficult to do. With modern TTL metering and dedicated strobes, your camera can automatically determine exposure for the combination of flash and ambient light. If you're using a digital camera, you'll be able to confirm your results on the LCD monitor.
Just slide your flash into the camera's hot-shoe, set the flash system for slow-speed sync and start experimenting. Slow-speed sync has various other names, like Night mode, depending on the camera manufacturer. It's designed to ensure that the camera's auto-exposure system will give you a correct ambient exposure, even though the flash is activated. Without it, your shutter speed likely will be too short and areas not lit by the flash will be too dark.
The lighting will look even better if you take the flash off the camera. When you position the flash away from the lens-to-subject axis, you increase the impression of your subject's three-dimensional form and texture. You'll need an extension cable to maintain the flash's electronic connection to your camera. Some camera manufacturers offer wireless flash systems, which work over longer distances than are practical for the cables. They provide TTL flash when the strobe is as far as 20 or 30 feet from the camera.
As it gets darker outside and your exposures run 30 seconds or longer, you'll have a chance to fire multiple pops of the flash to illuminate several areas in your shot. Use the flash's test button to trigger it as you walk around—you won't see yourself in the finished picture as long as you keep moving and make sure the flash head doesn't point toward the lens. Avoid recording your silhouette by staying well to the side of whatever you're lighting.
TTL metering won't work with this approach, but your flash probably has a built-in sensor for non-TTL auto. Make sure you match the ƒ-stop that the flash will use to the one set on your lens, and don't be afraid to experiment!