Usually, even for relatively small backgrounds, up to 10 or 12 feet wide, many pro's will use 2 lights on stands positioned on the sides to cover the background. It will help a lot if you can arrange for enough workspace to have the lights moved out far enough to allow their light to spread more evenly across the background. Adding diffusion or an umbrella can help.

In this example we are using two Tota-lights bouncing into white umbrellas. Notice how one of them flares into the camera lens. This is something to always be checking for. Its best not to have flare to begin with, but if you can't find a light position that avoids it, then clamp a lighting flag or some cardboard to a stand and position it so that it cuts the spill of the light into the camera lens.

If you get some spill from the background lights onto your subject you could either accept it for its effect or flag it off so you can light your subject completely separate.

Take a close look at the final image. Attractive as it is, it may be technically wrong in that it doesn't simulate the quality and direction of the light on the leaves in the background.

The background image has already been created, and it was created in a certain lighting environment. In order for the keyed subject image to believably fit into the background image, it has to be lit from the same angles the subject would be lit by if they were actually standing in the background image when it was taken.

In this example we tried to match the light direction and quality of the scene that is matted into the background. The sun's direction is indicated by the shadows on the window frames. There is no hair light. In this case it would have been an unnatural second source. Notice, the biggest light is not always the key light.

The green screen background is being evenly lit by two Omni-lights with 300watt bulbs and diffused with Special / White Tota-brellas. The barndoors were left on to block the direct light from the front of the light flaring the camera lens.

DP Light with a Special / White DP Brella and a 1000 Watt bulb is providing the fill to open up the shadows on the model. Again the barndoors were used to flag the front of the light. A combination of distance and adjusting the barndoors allowed us to get the fill level where we wanted

Omni-light, 300W, focusing

Rifa 44, 250W

DP Light, 1000W

If you must work in more restricted spaces, an alternate but less versatile method is to light the subject and the background with the same flat lighting.

With large soft lights the shadows will be minimized and you may be able to position your subject fairly close to the background. You will have to experiment to determine how deep the shadows can be and still work with your system.

Here we are using two 1000 Watt DP Lights with Special / White DP Brellas. You can make out the twin shadows on the green screen. Notice one is a bit dimmer because the light on the right is further back.

To put some depth back into the image try adding modeling and defining edges to the subject, with separate and more controlled lights, taking care to not get shadows on the background (unless you want them of course).

We added a Pro-light from behind on one side to try and match the direction of the sun in the background image we will use.

The final composite image. The light on the trunks of the trees was the clue to the direction of the natural lighting in the background. To be totally realistic the front of the model might not be as light as in this example. By making the model lighter we have chosen to draw the viewer's attention to her.