The following optional technique can help you create cleaner keys more quickly.
While creating the key, you can view the composite using a solid colour for the background in place of the back clip. This can help you see details in the image, such as colour spill, that you may otherwise miss. You can use the default colour (black) or select a colour using the colour picker.
To use a solid colour background:

Your key is displayed on a solid colour background in the Result view.