The Plugin Site

Try these steps:

Auto color - auto levels - auto contrast.

If that doens't work totally...select the objects, inverse the selection and then bring up curves to whiten.

I think that if you want to do knock outs, depending on which software you have ,white would be the last color I would use for this,
I think a color that is darker and has alot more contrast would be better.(dark blue or black)
After you have made your selection, select Select/ feather 1 pixil. then clean up your selection with what ever tool you choose to use. Put that selection on a new layer then select Layer/matting/Defringe, use the default here of 1 pixil, change if you feel you might like to.(don't be afraid to play with this setting), make sure the check box, show preview is checked. Then with your new background in place ,paste it. Then download this free filter..http://www.bmath.net/dc/.
I think you will be very pleased with the results
Photofix

Try Paint Shop Pro 9 or even 8's background eraser tool simple to use and on a single color or tint or tone background simple and direct much more so than in Photoshop