The purpose of the preflight printing process can be broken down into multiple steps. The main purpose of the preflight printing process is make sure that everything for the  job is correctly prepared for printing. It is very important to
Checklist

•  The artwork is designed at the size it's going to be printed.
• Does your document have the proper bleed setting?
• You've removed any extraneous elements from the pasteboard.
• Your "screened" elements are set at no lower than 15% opacity (any lower and they're practically invisible).
• You've supplied all screen and printer fonts used in your file and any image files imported in your document? If you properly pre-flighted and packaged your file, the answer should be yes.You can also choose to outline your fonts, if you're not allowed to share the font. However, we prefer having the actual font file so we can make changes to the text if needed.
• Have you verified that ALL COLORS used in your document used are set to CMYK values for proper separation? Please remove all unused colors from your document.
•  Make sure color graphics (eps, tiff, etc.) have been converted from RGB to CMYK and scanned at proper resolution (300 dpi). NOTE: Do not use compression when saving images. Save scanned images as .tiff files. Make sure all black and white images are set to true "Grayscale".
• Have you replaced large black areas with Rich Black (75% Cyan/50% Magenta/50% Yellow/100% Black) to make them appear more solid?
• Did you check the spelling and grammar of your document?
• Did a friend or co-worker check the spelling and grammar of your document?
• Is your page layout program supported by Action Printing?
• Have you backed up your files on your hard drive or other media in case of file damage? If not, please do so now.
• Have you given us a printed, physical copy? Please provide us with a hard copy either printed, faxed, or e-mailed. This way, we know what the finished project should look like and we can check for inconsistencies.

A Prepress Technician formats and proofs, they make sure that everything is ready and complete for printing.  A prepress technician has to has a great eye for detail. An average salary for a , a Prepress Technician is around $37,970 annually or $18.25 hourly. 





 Sources
http://inkd.com/earn/prepress
http://www.action-printing.com/help/the-time-saving-preflight-checklist
http://www.bls.gov/oes/current/oes515111.htm