- put your subject between you and the light source
- if possible, pose the subject to get an interesting silhouette
- whack up the ISO as high as it will go
- steady the camera on a tripod / firm surface / whatever
- take the shot!
the high ISO will introduce texture in the form of noise, so this technique gives you the possibility of getting a shot when there really isn't enough light.
consider introducing motion in the subject (experiment).
there is a every chance that this picture won't come out as a masterpiece, but we're talking about a situation where you have very few options (unless you're going to use flash), so why not take a chance, and ... take the shot!