First you need to meter the background.
Then "the most accurate way" with a flash meter, measure the flash at your subject. It is usually more pleasing if you have a 2:1 or 4:1 ratio of fill. 2:1 is a 1 stop less fill light then ambient, 4:1 is a 2 stop less light.
Without a flash meter it is more difficult unless you are real familiar with your flash's output. If your camera has a flash exposure compensation, you can drop it by 1 or 2 stops. My Canon A2 has this capability and it works quite well when the flash is set to TTL.
If done correctly you should not be able to tell a flash was used. [img]/ubbthreads/images/graemlins/cool.gif[/img]
If your ambient is too dark you need to use a slower shutter speed.(Dragging the shutter)
Shutter speed controls the ambient exposure.
Aperture controls the subjects exposure.
Hope this helps, If anything is incorrect someone please let me know, as I am in a lighting class right now and I'm learning also.
Rolando had a real good "Photo Tip" on here at one time on this subject. It may still be there.