THE DO'S & DON'TS OF PREPARING ART FOR OFFSET PRINTING:

MIXING COLORS:
Make up your mind what color mode your document is going to be printed in. If you are printing CMYK four color process then pick your colors using the CMYK color palette.
DON'T assume that the colors that appear on your screen will look like that when they are printed. Your monitor is projecting color on its screen in RGB color. It is trying to simulate what CMYK colors look like but because the light is projected and printed images are reflected light. Variations in how your monitors brightness and contrast are set will also effect how a color will look. The best option is to purchase a CMYK printers chart and compare the color you choose with the real printed item. Good art supply shops or your printer himself will be able to tell you where to buy a printers chart
If you decide to use spot colors for your print job you will probably use the Pantone color matching system. This is the most widely used color matching system in the industry.
Again DON'T assume that the colors that appear on your screen will look like that when they are printed. Pantone colors are pre-mixed flat colors and in many cases it is impossible for your monitor to simulate the color in RGB. For instance some of the colors are real metallic.

Pantone color charts are available at all Graphic art supplies. You most definitely need to look at the printed color on the chart and DON'T pick a pantone color from the screen unless you have years of experience in color matching.
Now here's a big DON'T..........DON'T use CMYK colors, Pantone colors and RGB colors on the same document and send it off to the printer. Why? Because he will have to convert everything to CMYK for you before he can print the job and you might end up with something entirely different than what you planned.
It's surprisingly easy to make this mistake. For instance you may have a company logo created using Illustrator, it is a vector image using text and graphics using the company logo Pantone colors. You import this logo and place it on top of your document. You then import and image from your digital camera and place that below the logo. If you import this straight from your camera it will be in RGB format. You then decide to color the text using then default color palette (which is CMYK) and there you go, you just mixed three different color modes on one document.

You can choose to export the whole document as a CMYK PDF file and yes it will convert everything to CMYK and in doing so may make the logo print very differently than the corporate Pantone colors that have been set. You should do all the conversion work yourself, picking the nearest colors from a chart and not the screen.

NOTE: Programs such as Scribus, In Design and Quark Express are essentially assembly programs. Typically the Vector Illustration (Logos,Line Illustrations etc.) are created in a Vector drawing program such as Illustrator, Corel Draw or Inkscape. The Photographic Images (and many of the special Text effects and artwork) are prepared in a Digital Imaging program such as Photoshop or The GIMP. Programs such as In Design, Quark Express or Scribus enable you to create multi page documents and add text and special effects with a high degree of accuracy. These programs typically place a linked Image in the working area of the document without adding the actual Image to the saved document. This enables you to be able to work on very large documents without slowing down the computer. The Images and artworks are only packaged when you prepare the file for the printer, this will naturally then create a much larger file. These programs are also marked for the high quality of the text that they produce. The alignment, paragraph spacing, line spacing and even spacing between individulal letters are of a very high standard...thats where they get their reputation. 

PHOTOGRAPHIC IMAGES:
DO: you should really open the Image in a photo editing program such as Photoshop or The GIMPand prepare the Image before you use it in a document you intend to send to an Offset Printer.
Take for example an original Image of 908 mm by 681.57mm at 72dpi. Most digital cameras shoot Images at 72dpi but the Image is very large at nearly a meter wide. Resample or Scale the Pixels per inch to 300dpi (which is the quality needed for Offset printing) and the size is now 218mm wide. This tells me that the image is of good enough quality to print at the most 218mm wide at 300dpi.
If the Image was 218mm wide at 72dpi then it wouldn't be good enough for offset printing.
Next you need to convert the Image to CMYK. Thers is a tutorial in thegimptutorials.net that covers this subject.

Now here's a big DON'T:
Make sure that the photographic images used in your documents are of a high standard. Typically Images taken from a Digital Camera should have been shot with the camera set on RAW. In other words using as little file compression as possible. JPEG files are compressed files, the more compression the more of the Image has been lost. Jpeg files are very small and take up less space on your memory chip or hard drive, but this comes at a price.
This poorer image will give a POOR result when printed by an offset printer.
There is a saying in the Printing Industry.
'Rubbish in, Rubbish out'
The really frustrating thing is that a 72dpi jpeg may look OK on your screen, but when printed will look terrible. Always zoom in to an Image and have a good look at it. Don't forget to check it's size and resolution.