How to get great-looking flash portraits at sunset

Turn off flash, switch camera to program mode.
Aim at the sky, but make sure not at the sun.
Hold the shutter button halfway down. This tells your camera to take a meter reading of the sky, so while that button is still held down, look in your viewfinder to see the f-stop and shutter speed, and remember what they're reading. For example, it read f/5.0 at 1/60 of a second.
Switch your camera to manual mode and dial in f/5.0 for your f-stop and 1/60 of a second for your shutter speed.
Note that now the sunset sky will look perfect, but your subject will be almost a silhouette.
Turn your flash back on, but lower the brightness or power output of the flash by around two stops, so just a little bit of flash fires, not enough to overpower the existing light, just enough to light the head and shoulders of your subject.
You'll have to fire a couple of test shots to get this just right. Bare in mind do not change the camera settings. They are perfect as is. Just lower or raise the brightness of your flash for the best quality.