As for more on color correction, please remember one important thing that 80% of designers (and salespeople) always forget: Unless you and your service bureau have the exact same make and model monitor with the exact same calibration settings, the colors are not going to be the same. With that said, apply that same theory to printing presses. I work exclusively with a printer who has 3 digital wide-format presses and each one prints differently even though they're the exact same model, etc. Also, the resolution (as Retrocity covered quite well) is very important! I design mostly billboards and large-format prints so everything we do is 400dpi, but there's nothing that irks me more than a client sending in a 3"x 5" 72 dpi photo or artwork. It drives me nuts how people don't understand relative resolution. Ok, before I start to sound too bitter :) I just wanted to add all that in. Good luck!