I'd think again about using flash... If you haven't been shooting long, chances are that you think of flash as the ugly white light that blasts the subject and leaves everything else to drown in darkness. But there are a few tricks to keep the (beautiful) natural light in your shots, even when you use flash. Give me (or us of course) a shout if you'd like to know more, but here's a few short hints:
-set your flash sync to rear curtain sync (check your manual)
This setting means that your flash will fire at the end of the exposure instead of the beginning
-set a longer shutterspeed
In dark conditions I start between 1/5th and 1/20th of a second. Together with rear sync flash this will let your camera suck in the (good) natural light before freezing motion with the flash. You will get motion blur, but your subjects will still be rendered sharp by the flash. Colours and ambient light will also be rendered much better.
-raise your ISO
To keep shutterspeeds at a reasonable length and still collect as much ambient light as possible, raise your ISO until you get a good mixture.
For an example click here: http://www.flickr.com/photos/brunoaxhau ... 5/sizes/l/
This image was taken in a pretty dark tunnel although the light was very warm and nice. Without flash the subject would have been hopeless to freeze but I also wanted to keep the ambient light. So I basically did the above, shutterspeed around 1/5th, rear curtain flash sync and ISO up around 800 or more (don't remember the exact settings, sorry)
Of course you can do longer exposures with flash set to front curtain sync (which would probably be the standard setting) but then the motion blur of moving subjects would be on the wrong side. (People would seem to be walking backwards etc.)
An external flash is nice for this of course but you can do all of this with a pop-up flash as well! In fact, for shots of parties etc all I carry is my D90 with a cheap 18-55mm and it's pop-up flash. Doesn't get in your way and works like a charm.
Hope this helps a little,