Do a little arithmetic before you scan. Planning to reduce the image to 50%? This doubles the resolution. Reducing to 33% triples the resolution, and so on.
A final resolution of 2 times the LPI works well in most cases. LPI stands for lines per inch, and for offset printing the LPI is usually 133 or 150. Final scan resolution means that you've calculated the change in spatial resolution when you size your original scan, so the formula is: DPI = LPI x %increase or decrease of original x 2
Clean up scans in a retouching program before using them. Adjust their size and resolution before placing them in a layout program.
Crop the image in a package like PhotoShop as close as possible to avoid having the RIP compute all that white space. Avoid using PageMaker or other layout programs ability to crop a scan, this feature merely masks an area and does not decrease the data that the RIP must compute.
Another tip is to add noise to an image to minimize the transitional steps between gradations. For instance if you have created a blend in your illustration package that has thirty steps, you can export the EPS file into an image editor and add 1 or 2 units of random noise with the noise filter to take away the hard edges between steps.
Line Art scans, since they can only be black or white, do not have the luxury of anti-aliasing (a technique that makes smooth transitions between black and white by adding varying steps of gray), and therefore are prone to having jagged edges around curves when scanned at lower resolutions than the final output device. Therefore, you should not use Line Art style scanning to scan photographs or textured art.
Line Art should be scanned at as high a resolution as the final output device to eliminate the jaggie effect. In other words if you have original art that is 5x7 and you are going to output at 5x7 on a 1200 dpi imagesetter, ideally it should be scanned at 1200dpi to achieve the best possible image. Note that if your original is larger than the final size, you should adjust your original scan resolution by the same ratio so as to not have more resolution than necessary.