One thing you can look at is the angle of your main light to the background. Is it falling directly on the background? Does the light cross the background? If it's a straight line from the light to the model to the background, then you need to get the model farther away from the background. The model should be closer to the light then the background.
If the light is striking the background at an angle, you can feather the light by using a piece of foamcore to block the side of the light toward the background.
Hope all that's understandable!
The main light here is to the image right, and has a piece of foamcore leaning up against it to keep it off the background. The fill light is to the image left, shooting in parallel to the camera. However, the model is far enough from the background and the light is low enough that the fill light falls off before it hits the background.