To reduce the effects environmental lighting has on your LCD monitor you can fit an LCD hood. By doing so, you'll be able to view the true colours of the image you're working on.
An LCD hood is attached to the top and sides of an LCD monitor and one which is specially designed for the job should be your first consideration. However, you can create your own hood with a little DIY project. Try cutting a piece of cardboard, plastic sheet, or polystyrene board to the size of the display then paint the entire surface facing the LCD monitor screen matte black to minimise light reflections.
Please make sure that the hood doesn't block heat release vents as heat buildup inside a monitor can damage the unit.
In the end, the hood simply needs to block environmental light from reaching the screen of the LCD monitor and not reflect display light back onto the screen.
If you're an EIZO ColorEdge monitor owner your model can be fitted with a special-purpose hood lined with black felt to suppress reflections. Visit the EIZO website for more information.