Design files in CMYK
When editing your files, make sure you use CMYK colors from the start. This way you don't have extra work correcting certain colors that can't be (correctly) converted to CMYK, and you are sure the final print colors do not differ from what you originally intended. Read more about the different color codes.
Converting RGB to CMYK is very easy. In Adobe Photoshop, go to Image> Mode> CMYK. In Adobe Illustrator, go to File> Document Color Mode> CMYK Color.