The problem with this is, your camera is pretty smart...but not that smart. Sooner or later you have to quit letting your camera tell you what to do, and you need to start telling your camera what to do. I want to teach you a tip in the manual mode.
|ISO 200, 1/200, F13|
Try this. Close or turn off your flash. If you have a DSLR camera...put your camera in the "auto" or "P" mode. Now point your camera at the subject with the sunset (or any backlight) in the background. Now press the shutter button down half way and look through the view finder. You'll see your camera's shutter speed and aperture settings...usually at the bottom of the view finder. Memorize those numbers. This is a quick way to meter the light.
Now switch your camera to manual mode (M). Set your camera to those numbers as a starting point, then take the photo with your flash on. You will see your photo is better exposed. Now that you have a starting point in the manual mode, try adjusting either the shutter speed, or the aperture (F-stop) until you get a perfectly exposed shot. The higher the shutter speed, or the higher the aperture value, the less light you'll let into your camera. Experimenting with these numbers will help you bring out the colors of the sunset.
If you have a point and shoot camera, try setting your camera in the "sunset" preset and turning on your flash. You'll accomplish the same thing. :)
Have fun experimenting!