Files should always be prepared in CMYK color mode, not RGB. If you are using RGB images in the design process, be sure to convert them to CMYK before exporting your final design file.
Be aware that RGB files that are converted to CMYK will have a slight color shift on screen and in print.
Be sure all grayscale images maintain their grayscale (monochrome) value, and that no other color values are added.
If your file is printing strictly black and white, be sure all artwork and images are in grayscale mode.