I wanted to do this post for quite a while, as I struggled on how to best prepare my files for professional printing in the beginning and thought sharing my experience and solution might also help some of you. I have found a way that works very good for my files and have been very happy with how the prints turned out be it cards or books. You are more than welcome to add your knowledge and experience in the comments section though. It would be lovely to hear what obstacles you have had or still face with the printing process.
When illustrating in mixed media, combining traditional with digital mediums, I work in RGB modus when taking the illustration to digital. This is simply because some features are not available in Photoshop otherwise and that the different printers use different printing profiles, which will result in a different print out quality when it comes to color correctness. When the illustration is finished I convert it to the color profile the printer I am to use uses. For European printers this is mostly the color profile FORGRA39 , US printers mostly use Coated GRACoL 2006. You will find the option to choose your CMYK profile under Edit -> Convert to Profile, which will open a menu with a drop down menu with lots of CMYK profiles.
The tricky bit comes right after this. As the printing process makes the pictures always a bit darker, I usually use the Levels tool to lighten it up a bit. Mostly I move the slider to +5, or rather 1,05 sometimes even 1,10, if it is a rather dark illustration. The slider will be set to 1,00 when you open up the Levels tool.
Then I use the Chanel Mixer tool to adjust the colors a bit. This is fairly important in terms of printing dark blues in order to make them look blue and not black. Each color has it's own menu selectable at the top and the color in use will be set too 100 and the rest to 0. In the Yellow, Magenta and Cyan menu I set the color 105 and the black to -5 for each color ( Yellow from 100 to 105 and black from 0 to -5, Magenta from 100 to 105 and black from 0 to -5, Cyan from 100 to 105 and black from 0 to -5). In the black color menu however I set the Cyan to +10 and the black from 100 to 90, the rest of the color bars remain at 0. Finally in order to make up for the little loss in saturation with the printing I go to Hue/Saturation and raise the saturation bar to +5 max +10.
This is not the way you have to do it, just something that has worked very well for me and also for the illustration you see right here, which was on the front of my last promo mailer.