First, make sure your house isn't too hot! Safety first! [img]/ubbthreads/images/graemlins/wink.gif[/img]
Then "drag" your shutter while shooting. Two ways to do this, one way is to not get the surrounding light of the area where your model is posed.
Now fire will always change shapes and color as it burns, including light intensity, so if you want to really see what you're fire "reads" on your meter, turn out the lights and fill your frame with fire only and take a light meter reading, if it's f/5.6 at 1/4 of a second, then shoot at f/5.6 at 1/4 of a second with your flash source. LIghts out when you shoot, except the modeling light lighting your model to help you see the light and focus. This affect will give you effect of a romantic night.
You would set your light source to meter F5.6 to about 2/3 stops over, say f/7.1, then shoot at 1/4 of a second.
Now if you want the ambient light, take your meter and make sure in the same scenario that if you shoot at 1/4 of a second that you will read at least f/5.6 or 2/3 stop less of the scene itself. If you have this, shoot away, lights on. Normally you don't have that much ambient light to worry about it...all fails, bracket, and if you shoot digital, you normally can see your results--the bottom line of it all, is you "drag the shutter" to a slow shutter speed, this give you a "fire" look not a "frozen" look, just like you would drag the shutter when photographing a stream of water flowing--if you're shutter speed is too fast, it looks like frozen water. Hope that helps, thanks, rg sends!