We have ours prepared by our signage sponsor and adhered directly to the bumper material. The numbers are printed with a background that closely matches the color of the bumper's material. They stand out well and hold up well. We have some extras made to keep in the pits in the event we have to replace them during competition.
View the red here:
http://casey99.com/Events/Robotic_Champs/dsc_2518.htm
This is the blue: