Keep in mind that when you are working in low light situations - particularly indoors - that you will inherently have to deal with shutter speed and/or depth of field issues. If your subject is lying down, shutter speed can be less of an issue because he/she (they?) can hold still enough over a second of two so there should be no discernible blur in the resulting image. On the other hand, if your subject is standing, squatting, or doing anything ..... shall we say, "active" ..... then shutter speed might be a problem without a lot of light. Slow shutter speeds also preclude hand-holding the camera, making a tripod a must.
Of course, the first thing you could do is increase your aperture (lower the f-stop). The problem there is that you are losing depth of field. For some shots this could be highly desirable and result in an artistic shot, but oftentimes it means that some part of the image you wanted in focus will not be.
If you shoot digital, you can also adjust the ISO up so you maximize the shutter speed and depth of field. Going too high will make your photo noisy, but an ISO of 400 or even 800 on a lot of cameras is probably going to be acceptable.
The remedy for all this is light - lots of it and diffuse. You can always reduce light if you have too much, but getting more when you need more can be a pain in the ass if you're all ready to go and it's just too damned dark.
