While Photoshop files intended for use in full-color print jobs will ultimately need to be converted to CMYK color mode, working in RGB color mode means more filters for you to use, and a smaller file size, which allows Photoshop to operate more efficiently.
Knowing these benefits, wouldn't it be nice to work in RGB mode, but see the effects of your work in CMYK mode?
Here's how you can do just that in real-time. While your image is open onscreen, go under the Window menu to Arrange, and choose New Window. This will open another view of your existing document. Press Command-Y (for a PC: Control-Y) to show a CMYK preview of your image, then return to your original document and edit as normal. The changes you make in the RGB window will be updated in the CMYK window.
Your finished file will still need to be converted to CMYK mode for use in full-color print production. If you need assistance with this final step, please contact us - we'll be happy to help.