I'm with Photo_Chick on this. Your first best choice is diffuse window light. Not sun streaming through the window light; you want cloudy day or north side of the building window light. It also helps to have a reflector opposite the window to help fill in the shadows. That can be a white sheet or a large, white painted piece of cardboard.
Using lights makes life easier because you aren't dependent on the light coming from a window. Like everything dealing with photography, you get great bang from the buck for your first piece of gear and rapidly diminishing returns for each additional piece. In other words, a single flash is awesome compared to none; a second flash is nice; a third flash helps a bit; a fourth flash makes a trivial difference, etc.
My basic layout suggestion would be to set your light source (flash or window) in front of and to the side of your subject. If clock terms, if your subject is at the center of the clock and the camera is at 12 o'clock, you want your light somewhere around 1:30 or 10:30. On the opposite side, you want your reflector. Position it so that it best fills in the shadows.
You don't want your flash on your camera. The cheapest way to get it off is a remote flash cable. A nicer way is a flash commander for your camera - something like an ST-E2. It won't make your pictures better; it's just easier to use.
You don't want your flash naked. It's too small. Shoot through a white umbrella. You can use a thin white sheet if you have to. I've even heard of people using shower curtains. The main thing is that you want the light source to be bigger, so shoot your flash through something that will work like a large light source. You can also move everything closer, which makes it bigger relative to your subject.
That's the basics - large, diffuse light source in front and to the side of your subject and a reflector or fill light to fill in the shadows. You can tweak a lot of stuff from their (balancing ambient and flash, adjusting the color of the light, adding additional lights, etc), but that is where you want to start.