Work in the Right Software
Prepare your art in the program that is best suited for the task. Illustrator® is a drawing program, ideal for logos, packaging, posters and single panel layouts. Photoshop® is a pixel based (raster) program that lets you size, color-correct and manipulate scanned images such as photographs and flat art. Neither is intended for multi-page documents. For that, use a page layout program such as QuarkXPress® or InDesign®
Start with the Best Art Possible
Despite all the advances in graphic software and printing, the adage, "garbage in, garbage out" still applies. The better the original, the more latitude the printer has in achieving the results you want. Use photographs that exhibit the full tonal range. Don't convert a color photo to make a black and white reproduction, if a black and white original is available. Start with the best you can get because each step results in a generational loss in quality.